Absolutely do not buy a phone with a glass back. Rumours are that the X4 has a glass back. This is hands-down the most ridiculous phone feature ever created. I bought an HTC U11 and it is so ridiculously slippery I can't hold it. The very first time I took it out of the box and put it on the table, it slid off like it was haunted. A day later it slipped again and crashed to the floor and broke the screen and back. Do not fall for this ridiculous trend. My old Moto X PE has the customizable back and I chose the hard rubber, that's the minimum you should have on a phone.
There should be a plus model, with 5.7 inch fhd display, same chip, same camera, 4 gb ram, 64 gb rom, and 4000 mah battery.