gengar is more specially attacking, while absol is more physical. absol is weak to: fighting, bug, fairy. gengar is weak to: ghost, dark, ground, psychic. absol is faster (if I remember), but also alot quishy-er? is that a word? gengar is good for koing fairies, ghosts, and psychics. absol is good for oking ghosts, psychics, and (if psycho shift is invloved like I do on my absol micheal) fighters, and poison types. they are both very good chioces, but gengar is quite a bit more meta. gengar has shadow tag, so theres no escaping (unless the oppenet is ghost type as well), and absol has magic bounce, good for anti-spike/stealth rock plays.
edit: I missread the title. I thought it said gengar. gardivour plays a simular role, but it is a bit slower, and its fairy-psychic typing gives it no 4x weaknesses, it has accsess to alot of very powerful moves, like dazzeling gleam and moon blast, along with revovery moves, like draining kiss. and yes, I ment psycho cut not shift.