Though my vote is for the 1520 (as I own a 1520.3 myself), there will be an HTC one M8 variant with GSM for global use within the next couple months. It might be worth waiting to see how that might serve you as an alternative.
Sent via Lumia 1520
I use the exact same Imak cover. It is amazingly thin and grippy. Indeed better than an Incipio feather and Niilkin that I also own.
Sent via Lumia 1520