I had the choice between the L1520 and the M8 for Windows on AT&T. Trust me, I spent tons of time playing with both. I chose the M8, but let's look at general plus and minus of each device on T-Mo.
First off- you know some of the advantages you will lose using an unlocked phone on any carrier in the US. Put that in the "plus" category for the M8.
Next, I think the M8 takes too much crap for it's camera. It is fine. The lower res is a problem for long distance shots, but in all other categories it is fine. However, I am silly enough to not realize the L1520 isn't among the best shooters in the industry. Throw in the improvements in an already excellent camera with the latest update, and it is a big time advantage to the L1520. Put that in the "plus" for the L1520.
Screen wise, you get a slightly higher pixel density on the M8. The L1520 is a great screen. You will not hurt your experience at all. But there is a sharpness on the M8 that comes with that higher density that is noticeable. Plus- M8.
Audio is not as wide of chasm as I thought, First off- the audio and speaker phone options on those screens are far and away better on the M8. Sorry, but it isn't even close. But with headphones on, there is little to no difference at all. I do not think the L1520 is better, but I do not think the M8 is either. Seeing how most people use earbuds or headphones the audio advantage is not really that great. It is still a plus to the M8, but not really that much of a plus.
Support- this isn't even worth mentioning. L1520 in a walk-over. MS will support this monster, HTC needs to prove it. HTC has been better, especially on their Android, and has brought some nice tools over. But the updates, improvements, and promised support to all Lumia devices has been proven. Big, huge, MONSTROUS plus to the L1520.
Internals- why not? It's a consideration. The L1520 is a well powered device and Windows phone (and soon Windows 10) will run seamless on it. But, points are points. The M8 has more powerful internals including graphic chip, processor, and screen. It performs slightly better on tasks and commands. Not enough to make any real difference on real life use right now, but it may make some difference in the future. Small plus to the M8.
As mentioned by c0wb0ycliche above, there are a TON of cases and accessories for the M8. If it fits the Android M8, it fits the Windows M8. That is nice. The L1520 has options, but not nearly as many. Again, not a huge plus, but a plus for the M8.
It all depends on what you want. If camera and support is something you need, than the L1520 is your best bet. If accessories, external audio, and more up to date internals is a concern and you are willing to accept the device you are buying for what it is now and not what it will be in a year along with the advantages of a W8.1 device designed for T-Mobile use, you will love the M8.
Any other measuring stick- screen size, looks, colors, how it feels in your hand- are all personal choices and you should not listen to anyone else when deciding on those features.