I vote metal. Don't want glass. It's a fingerprint magnet, is very fragile, scratches easily, and allows for a sealed back battery. Would rather sacrifice wireless charging for a metal back phone. I really enjoyed the HTC M series with metal back. Enjoyed the metal removable back of the V20 even more. Plastic/rubberized plastic is my second choice but I don't like how it makes creaky sounds sometimes (Galaxy S5 for example).