-Amazing SpA of 170
-Very fast with 130 Spe
-The outstanding ability of Shadow Tag that traps the opponent
-A nice movepool to work with, having Shadow Ball, Sludge Wave, Dazzling Gleam, Giga Drain, Encore, Substitute, Perish Song, etc.
-Awesome 165 Atk
-A very good ability in Prankster
-Access to Destiny Bond, Will-O-Wisp, and other good utility moves
-Sub-Par stats outside of Atk
-Awkward mix of stats and being better for utility
Overall, Mega Gengar is far superior.