Both are great, and could be very useful when battling. If I were you, though, I would go with Gengar. If you have Snorlax out on the field, you could easily switch into this Pokémon when you're expecting the opponent to strike fighting-type attacks. A good moveset would be Shadow Ball, Focus Blast, Sludge Bomb, and either Substitute or Hidden Power Fire. For EVs, try 252 Speed, 252 Special Attack, and 4 HP. Go with Timid nature, and make it hold onto Life Orb. d: