Pretty much, yes.

There are still small, regional GSM carriers in a few places, but they're few and far between, and last I looked, none of them appeared to offer much in the way of data plans. I don't know that San Francisco is home to one of those in any case.

Beyond that, we have a few MVNOs (Simple Mobile being one) that resell either T-Mobile or AT&T's network. They're less likely to have roaming agreements in place to allow you to roam onto the other network though, but that may not be a concern in San Francisco. The coverage there should be decent with either AT&T or T-Mobile.