If the roadmap from tmonews is true there wont be any new WP's on Tmo for awhile, just androids. And if they continue the trend with the Radar & 710 they wont have the high end offerings. I switched and am enjoying the 900 immensely.
Lumia 710 so you can use their 3G HSPA network, or wait and see. Lumia 710 will get you much of what you like about the 800 and 900, minus the sexier chassis. Only real downside aside from that is you're limited to 8 GB of memory. Should be adequate for just apps and data, but if you want to carry around a lot of music it won't be sufficient.
Lumia 710 - 3.7 in No AMOLED, 8GB, Not as attractive. Does have 3G on 2100/1700 and 1900.
Lumia 800 - 3.7 in Pentile, 3G only on 1900
Lumia 900 - 4.3 in, 3G only on 1900, LTE on 2100/1700
T-Mobile currently operates 3G on 2100/1700. Their LTE will be on 2100/1700 in 2013. They will also have 3G on 1900 in most markets, BUT NOT ALL, end of 2012.
Doesn't look like T-Mo has much of a plan to invest in Windows Phone, judging from their roadmap.
The Radar is the best WP device on that network.
I went through several lousy-quality 710s before having my "unchangeable, uneditable 5-device limit now that Zune is broken" experience. I'd suggest avoiding that handset.