I have used both T-Mobile and Sprint's data service. I had a Treo 180, a Merlin data card, and a T-Mobile Pocket PC Phone Edition, and used all of them to access T-Mobile's GPRS data network.
A couple of months ago, I bought an iPAQ 5555, with the PCMIA Expansion Pack Plus, and a Sprint CF2031 data card. I use the CF2031 in both my iPAQ 5555, and my notebook computer, swapping between the two depending upon what I want to do.
I have been very pleased with Sprint's network. I
really get speeds roughly double the speeds I got on the GPRS network of T-Mobile. So far, Sprint's coverage where I travel has been better than T-Mobile's as well.
Before agreeing to a contract with Sprint, I was pretty reluctant to go back to Sprint. I had used Sprint for voice, a few years ago, and had some bad experiences with coverage and customer service. Recently, I have called Sprint tech support a couple of times for help with my data service, and received good support. I think Sprint may have really fixed some of their customer-support issues.
You can read an excellent discussion of the CF2031 and Sprint coverage at
http://www.mobilecomputingsig.com/forums/index.php?