If you're going to be flying that much, I say pick one and stick with it. With some clever routing, you can get elite status (totally worth it). Get the airline's card to get some bonus miles, and w/ US you'll also get zone 2 boarding if you have their card (not sure about UA).

I'm new to all this but I'm finally getting why people are so excited about Starwood brands (Westin). I'm a Hilton person from awhile back, but if I was just getting started all over again, I'd go Starwood (I'm starting to transition my client meetings from Hilton to Starwood).

Enjoy Philly while you're here!