Just buy the Z2 (model 6503) from eBay. It works on T-Mobile & AT&T. It has all of the frequencies for any GSM carrier in North America. I routinely get 40Mbps down/7Mbps up on AT&T LTE on my Z2. The phone is awesome.
If anyone is considering the Z2, I can make it really simple for you. There are really no downsides to this phone. Spec-wise, it's the best. It has a 2.3Ghz CPU whereas the S5 has a 2.5Ghz, but the GPUs are both clocked at the same speed, so your gaming should be awesome. I have literally never seen the Z2 lag or stutter. The screen is awesome. The speakers are on par with the HTC One 2014. The camera is great. It's waterproof and dustproof. It looks fantastic and has a very premium feel. If you're looking at hardware, Sony takes the crown.
So what it comes down to is software and updates. Do you want a stock-like experience? Get the Z2. Do you want heavily modified UIs? Get the S5 or the One. The unlocked version of the Z2 has absolutely no carrier bloat, no tethering restrictions, and no carrier standing in the way to slow down firmware updates. Someone asked me yesterday if there were any negatives to my Z2. I mentioned a software bug that ruined a good app built into the phone. I woke up this morning and that very same app has a bugfix update. The app now works perfectly. There are no negatives now.