I love doing the cost comparisons for people on stuff link this. For example, the 925 (T-Mobile) is MUCH cheaper to own than the 1520 (AT&T)... Buying them outright, sure, the retail is $50 different, but the actual cost of the devices is closer to $600 than $50. This is why AT&T sells the phones for "less" than T-Mobile does.
T-Mobile Unlimited Plan - $70 a month plus $0 up front and $21 per month for 24 months = total 24 month cost $2256
AT&T Next 4GB Plan - $95.00 a month plus $0 up front and $29.25 for 20 months = $2865
AT&T Contract 4GB Plan - $110 a month plus $199.99 up front no payments = $2840
Now, on a side note, I grabbed one of these for my apartment where I have 1-2 bars and zero in a few places. I stuck the antenna in my window and the box about 15 feet from it. I instantly went to 3-4 bars on my 810 and full bars on my 925. My wife's 920 on AT&T had similar boosting, but she already had 3-4 bars, so the impact was as dramatic. The other part that was awesome was that it went from 2G data in most areas to 4G. It really works...
GetSmartphones.info - Smartphone accessories from Amazon and more! - zBoost YX545 SOHO Dual-Band Cell Phone Signal Booster for Home and Office up to 2,500 sq. ft. of Coverage
Given that I have no idea what you have in your pocket right now and that you only needed a T-Mo phone, I would run down to my local store and grab a Lumia 521 for the short-term and a back up device. The 521 appears to be receiving all the current updates, it's solid, and performs very well. Then you can start to save up that cash to grab the next device (maybe in a few months T-Mo will be running out their first 8.1 devices?) that strikes your fancy. Either way, a used 810, a new 925 or 521, you should be good to go for a while.