AFAIK, all the blackberry specific features(mainly push email, etc...) are pretty much the same across the board, as they are just rebranding RIM services. i'm not 100% sure on that though.
that aside, you need to decide what is important to you. data speeds? coverage area? network type?
us cellular probably has the best coverage in this area, but definitely not in others. verizon has the best data speeds. at&t has the most compatible phones for international usage(they run on GSM).
about a month ago, i spent a lot of time researching my next phone/provider, so a lot of this stuff is still pretty fresh in my head. if you can be more specific about what you want, i can probably get more specific in my answers.
for the record, i ended up getting a motorola q on the sprint network because i got a nice discount on my plan. i know someone who works for sprint and got hooked up with the employee referral rate. i decided that i didn't want to pay an extra ~$30 or so each month for the BB services. just wasn't worth it for me.