As long as you do not rush for having 4G LTE on TMobile you will probably be good to go with 929. I do not know how good is HSDPA on TMobile and if there is really a need for you to have LTE, but if not I would recommend to wait when 929 is released for others to post their thoughts about it before buying. 928 works ok on other networks but only because of Nokia's Access Point app, without it the internet would not work.
This is incorrect. The GSM bands are unlocked because when Verizon purchased their 700mhz LTE spectrum nationwide the FCC mandated the GSM/HSPA+/LTE bands remain unlocked. The issue with LTE working on T-Mobile is about incompatibilities between LTE bands. Verizon hasn't officially launched their AWS (aka T-Mobile compatible) LTE network, though I can attest that it is up and running in parts on NYC. I have a 5S and a 928 on Verizon but I switch them between TMO and VZW, and while the 928 has the hardware for TMO's LTE bands, it needs a firmware update to use it. The iPhone works perfectly on T-Mobile's network.
One thing to note, Verizon WP8 hardware has SMS issues on T Mobile, it will only send when you drop to the 2G network.
If Verizon launches it with AWS support out of the box then you'll be in luck. If not, just hope that you're in a refarmed area or you'll be stuck on EDGE. WiFi calling won't be an option as well.