The 925 is still a solid device, I'd go for it, but that's knowing I sprang for a 920 late in its lifecycle instead of a 520 and waiting to see what else popped up.
The 521 is probably the best choice if you're looking to upgrade once the next volley of phones roll out with 8.1 in a couple months. However whether they will make way to T-Mobile quickly after launch is anyone's guess. So you may end up with the 521 longer than you anticipated. But thats not necessarily bad, its my sister's main phone and probably will be for a while. Don't think she'll want an upgrade.
Also worth noting that looking at unlocked phones increases your options at the cost of LTE and some 3G depending where you live.