Do you still have unlimited data for $30 from AT&T? If so, keep AT&T. I've had an iPhone since the original launch, and the unlimited data carries over forever until you decide to change your data plan. Based on that, I'll be on AT&T for awhile. Also, only AT&T's network can take advantage of the HSPA+ speeds that iPhone 4S can do.
I think the general rule of thumb (if you aren't grandfathered into unlimited data) is to look at your phone use. Are you a heavier caller? In that case, Verizon is probably your best bet since their network is wider. If you are a heavier data user, AT&T can offer higher speed connections plus the ability to talk and use data at the same time without wifi. Of course, this assumes that neither network has serious issues in your area. If that is the case, that will obviously affect your decision.
Also, if you don't have but really want unlimited data, you might want to look at Sprint.
See my post two posts above on how to look up your shipping info directly on UPS. Apple is just being lazy about getting shipping emails out promptly.