Well really, it depends on your team.
The most notable thing about Spiritomb is its typing. Ghost/Dark. It gives it no weakness and useful normal, psychic and fighting immunities. It has great defensive stats that allows it to easily support. It lacks a good movepool but is still usable. It sadly has no realiable form of recover other than pain split and rest. It does good support moves like Will-O-Wisp, Toxic, and Hypnosis.It can even go offensive with calm mind to boost that shabby 92 base Sp.Atk.
Mismagius has great offensive stats. 105 base speed allows it to out speed all the 100 base speed pokemons like Victini, Zapdos, Mew, Celebi ,ect. A 105 base Sp.Atk paired with Nasty Plot and Calm Mind makes a devastating combo. It also can support with that 105 speed with Taunt and Will-O-Wisp. It also has levitate to switch into those high powered EQs. It has frail defenses so it can be easily revenge killed with a scarf user or priority (if weakened).
So basically, Mismagius for Sweeper and Spiritomb for support.