The Radar and Lumia 710 are the only two Windows Phones that T-Mobile stores sell right now.
Other WP7's that will run on T-Mobile's network: Dell Venue Pro and HTC HD7.
Lumia Pros: Second gen processor @ 1.4 GHz, Clearblack display, Nokia quality, Nokia Apps (Drive, Transit, etc), 4G capability (HSPA+, not true 4G)
Lumia Cons: No FFC, 3.7" screen is a little small, 8 GB of storage is limiting
Radar Pros: FFC, S-LCD display is nice, HTC Apps (Sound enhancer is great!), 4G capability (HSPA+, not true 4G)
Radar Cons: first gen processor 1 GHz, 8 GB of storage is limiting, some think Bing tap directions are limiting, 3.8" screen a little small
HD7 Pros: 4.3" display, HTC Apps (Sound enhancer is great!), 16 GB of storage
HD7 Cons: 1230 mAH battery is too small have to upgrade, 1 GHz first gen processor, 3G device, some think Bing tap directions are limiting
Dell Venue Pro Pros: (Not sure about this one because I don't own it/haven't used it) 4 in display is a good size/LED displays are better on battery life, full QWERTY keyboard
Cons: haven't used it myself, but it has the older processor, it will be thicker because the QWERTY keyboard, etc., 3G device
Those are your choices with T-Mobile, the 710 is a solid device, but I wish the screen was slightly larger and since Skype is coming, I wish it had a FFC. Also, I usually think complaining about space @ 16 GBs is a little over the top, 16 GB is a lot of content, but 8 I understand, between the OS and even a decent sized music collection it seems limiting. Other than that I see no reason it wouldn't satisfy your needs!