Comcast Spectacor has fired another salvo in the long-running public relations battle with the Sixers by proposing an alternative to an arena on Market Street.

At last, the Sixers and Comcast Spectacor have found something they can agree on: East Market Street needs a major intervention.

They just have radically different approaches for administering a cure.

Ever since the Sixers' owners announced their intention to abandon Comcast Spectacor's Wells Fargo Center and build their own arena on Market Street, they have portrayed the project as much more than a profitable real estate move. The arena, they keep telling us, is the last best hope for reviving the Market Street retail corridor, the only way to salvage what's left of the Fashion District shopping mall, and keep the lights on in SEPTA's Jefferson Station.

Initially, there was a seductive logic to their argument, especially in the absence of any meaningful planning by the city. The last time the Department of Planning and Development undertook a comprehensive study of conditions on Market Street was 2009. The iPhone was still a slick new gadget, online shopping was in its infancy, and workers dutifully trudged into the office five days a week.

But now Comcast Spectacor has stepped into the planning void with its own vision for Market Street, and this plan is seductive, too. Instead of plunking down a four-acre box on the edge of the fragile Chinatown neighborhood, the company would help convert the mall into lab space for medical research, turning the existing structure into an eastside version of the booming life sciences district that has sprung up around 30th Street Station.

In many ways, these strategies represent two poles of post-pandemic urban planning. As the rise of remote work has emptied out downtown office districts, cities have been forced to look for other ways to lure people into the center. Many believe they can attract people by turning their downtowns into playgrounds, where we eat, drink, play mini golf, throw darts, and come together for arena-size spectacles.

Others hope to reorganize their traditional centers around places where we create wealth by inventing things -- places like the West Philadelphia lab building where two University of Pennsylvania scientists developed the techniques that paved the way for the COVID vaccine and won them a Nobel Prize.

Both entertainment and knowledge production are necessary components for a thriving, 21st century downtown, along with housing and traditional offices. But which of these two proposals would help Market Street more?

This is a different question from the one explored in the quartet of city-sponsored arena studies that were released two weeks ago. Those reports primarily looked at how much money the Sixers arena would generate for the city and how the arena's presence would impact Chinatown.

While the reports concluded that the Fashion District's future looks grim, none evaluated whether the arena -- or the bars and restaurants that might follow in its wake -- are sufficient to give Market Street a new purpose. At the same time, the community impact study made it abundantly clear that the arena would devastate Chinatown.

I've always been skeptical of the claims that an 18,000-seat arena would be enough to sustain Market Street. Based on the Sixers' projections, the building will be dark more than 200 days a year. Large destinations that are occupied intermittently have a deadening effect on their surroundings. Just look at the parts of Arch, Race and Broad Street bordering the Pennsylvania Convention Center.

If the arena doesn't solve Market Street's problems and simultaneously destroys one of Philadelphia's most unique and vibrant neighborhoods, what's gained?

In contrast, lab buildings are more like offices. They're active five days a week, week in and week out. Comcast Spectacor's master plan calls for a mixed-use development that could include street-level retail, a community health clinic and library, along with 400,000 square feet of lab space.

While I wish there was more retail at street level, enough would be going on to attract visitors to Market Street daily. Like the Sixers' proposal, this one includes a modest apartment tower on the former Greyhound bus terminal site.

Skeptics note that there is a huge surplus of lab space around the country right now. That may be true nationally, but not in Philadelphia, according to John Gattuso, a commercial developer currently building the city's largest facility for life science research on the Drexel University campus. Because Philadelphia's stock of lab buildings is relatively modest, "the mid- and long-term outlook is positive," particularly in the area around the Jefferson Health campus, argued Gattuso, who has worked for Comcast Spectacor as a consultant.

It should be said, however, that the master plan is purely a speculative exercise.

While Comcast Spectacor has shared the details with Mayor Cherelle L. Parker and neighborhood groups, it has not offered to spend its own money to implement its proposals -- unlike the Sixers, who have said they are prepared to pay for most of the $1.5 billion arena themselves. Instead, Comcast Spectacor says it would act as a "convener," or facilitator, to help a developer acquire the mall and fill it with tenants.

It's pretty unusual to create a master plan that you don't plan to execute. In a carefully worded statement, Daniel Hilferty, Comcast Spectator's CEO and chairman, said the company isn't trying to compete with the Sixers for the rights to redevelop the mall. Rather, Comcast Spectacor produced the master plan because it was interested in putting forward an "alternative" in case the arena project falls through, he said.

All this parsing suggests that Comcast Spectacor's master plan is just another salvo in the long-running public relations battle between the two entertainment giants.

However suspect Comcast Spectacor's motivations might be, the ideas in the master plan are worth taking seriously. The company hired Ennead and Sasaki, two first-rate design firms, to prepare the plan, and their work was overseen by William McDowell, a consultant who has worked for the Redevelopment Authority, Barnes Foundation and National Real Estate Development, the company responsible for the extraordinary mixed-use development on the 1100 block of Market Street.

Like National's East Market development, this master plan is grounded in a strong urbanist sensibility. While the Sixers arena would expand the footprint of the mall's westernmost building, the labs could fit within the existing structure. Comcast's master plan outlines a strategy to strip off the mall's unappealing facade and replace the blank walls with large windows, giving it the character of a traditional office building.

Best of all, the master plan offers a way to cut the three-block-long mall down to size. Right now, its three sections are connected by sky bridges that block sight lines and cast dark shadows on Ninth and 10th Streets. If those two sky bridges were removed, it would restore the light to the city streets and open up views of the magnificent Chinatown gate on 10th Street.

The life science project could be beneficial for Chinatown in other ways. Not only would the labs draw some workers from the neighborhood, researchers are likely to patronize Chinatown restaurants. John Chin, who runs the Philadelphia Chinatown Development Corp. and who has spoken out against the arena, described the Comcast Spectator proposal as an "appropriate fit" that would not "overburden" the neighborhood.

SEPTA could also come out ahead, since researchers, like other office workers, tend to rely on transit to get to their jobs. The impact of the mall conversion on Jefferson Station would be negligible, and SEPTA would not have to shut it down during construction. One of the troubling aspects of the Sixers' plan is that it is likely to require the installation of steel columns and beams that could alter the character of the light-filled transit hub.

Many arena supporters like the ability of suburban fans to get to the venue by transit, since Jefferson Station is served by more rail lines than the Sports Complex. The city reports say that the Sixers could potentially get 40% of fans to travel to games by transit, up from the current 20% at the Sports Complex.

But what arena supporters forget is that the demand for parking won't disappear if the Sixers move to Market Street. Even if 40% of fans take transit to events, another 40% are expected to drive, the city reports noted. Most will park in the 29 garages and parking lots that currently surround the arena. That means those 29 parking facilities are unlikely to ever be developed for other uses.

Comcast Spectacor still believes it can convince the Sixers to return to the Sports Complex, either as a tenant in the Wells Fargo Center or a part-owner in a new arena. But after this scorched earth divorce, it's hard to imagine the two reconciling. New Jersey has tried to exploit the situation by offering the Sixers a building site in Camden and a whopping $400 million in tax credits for the arena. But this hardly seems like an improvement over the Sports Complex, since the Jersey site would be more difficult for both drivers and transit riders to access.

There are plenty of "ifs" in Comcast Spectacor's plan, too. Who knows if the mall's owner, Macerich, would agree to sell the entire mall to a developer. But after two contentious years of being told that the Sixers arena is the only way out of the morass on Market Street, Comcast Spectacor has given us one tangible thing: A choice.