It's hard to say which PA NFL team has the largest fan base. We do know that Philadelphia is a bigger city than Pittsburgh. However, the Steelers' history of accomplishments trumps the Eagles' history every time, which suggests a larger nationwide fanbase for the smaller city's team.
In 2019, Professor Michael Lewis, at Emory University, released a study of the best fanbases in the NFL. He ranked all 32 NFL fanbases based on things like fan equity, social equity and road equity.
The Steelers were at No. 5 and the Eagles at No. 3. The Dallas Cowboys landed up top.