T-Mobile may not give Windows Phone their highest priority, but as far as I am aware, my Lumia 925 has all of the updates for 8.1. I also like Wi-Fi calling.
Also, if you have a desire to obtain a Lumia 950 through T-Mobile, notifying T-Mobile of your preference may not do a lot of good, but if enough people follow suit, it will be better than doing nothing at all.