Let's be real here—the HTC One M9 was not a good phone, but the HTC 10 is. The only problem is that it's very expensive at $700. Now, HTC is knocking $100 off the price through the end of July, but the deal is only valid through HTC's online store. This probably would have been a better starting price.
At $599, the HTC 10 is a more attractive option. That makes it cheaper than the Galaxy S7 variants sold by carriers or unlocked. You have the option of picking up a carrier version of the HTC 10 or the unlocked version. There's no AT&T device, but you can get Sprint, Verizon, or T-Mobile. The unlocked phone is GSM/LTE, so that's the one to get for Tmo or AT&T as the price is the same.
In addition to the phone deal Alternatively, if you don't buy from HTC directly, HTC will offer $100 in credit for accessories if you can provide proof of purchase.
This article previously implied that the $100 off deal and $100 accessory credit could be combined. They cannot. The $100 off deal is only available through purchases made on HTC's website, and the $100 accessory credit is only available if you buy from someone other than HTC. Sorry for any confusion this might have caused. (Thanks, Thomas!)