Max typing speed: no significant difference. (YMMV, usually Dvorak at least as fast as Qwerty)
Comfort: Dvorak is much more comfortable.
Sustained typing speed: Dvorak is better due to much lower fatigue / strain.
Which layout should you learn?
Learn both. Dvorak for efficiency, Qwerty for compatibility (e.g. handphone, random computers).
One after another, not simultaneously. Probably Qwerty first.
My personal speeds:
Dvorak: 110 wpm (10-race average) on typeracer.com
Qwerty: 80 wpm (used to be >100 wpm when active)
Nowadays, using Dvorak exclusively. Qwerty occasionally for fun / convenience in rare situations.
Useful on Windows: Ctrl+Ins for copy, Shift+Ins for paste, Shift+Del for cut.
Bonus: hljk with Vim actually maps nicely on Dvorak.