I will also be buying the Lumia 925. It is aging very well, as it was already mentioned the only downside is the 16 GB nonexpandable storage. But it still has a GREAT camera despite being somewhat old (Imo, only beaten by 1020, 1520, and iPhone 5S. Maybe LG G2). It is probably the best 92x device considering its thin and attractive design. I'm not saying the 920 and 928 look bad, I actually prefer that typical Lumia design, but they're too thick imo! That's why I'm getting this phone (although, I admit, if the 1520 was available on T-Mobile I would go with that).
I wouldn't worry about not getting WP 8.1, I would just worry about how long you'll have to wait. Fortunately, though, T-Mobile doesn't seem to be as bad with updates as AT&T/Verizon are.
I am not happy with the limited availability of cases/accessories for the 925, though. It seems that most manufacturers are not making cases for this Lumia.