How about posing a different question, like would Rotom overtake Spiritomb in terms of effectiveness?
Rotom vs. Spiritomb:
Stats:
Rotom:
50/65/107/105/107/86
Spiritomb:
50/92/108/92/108/35
Minuscule defensive difference. Spititomb has 2 actual stat more Def/SDef. Big whoop. Spiritomb has much higher attack, lower initial SA, and crap speed vs. Rotom's mediocre number.
Weak/Resist:
Rotom:
Weak: Dark, Ghost
Resist: Steel, Flying, Electric, Poison, Bug
Immune: Normal, Fighting, Ground
Spiritomb:
Weak: None
Resist: Poison
Immune: Normal, Fighting, Psychic
Spiritomb has no weak. Rotom has a load of resistances.
Movepool notables:
Damage:
Possessed Fridge can Boltbeam. Fan can employ parahax, Wash does great in rain. Cut and Heat are marginal in comparison, but still there. Cut is easily the most awesome looking.
Spiritomb has priority in Sucker Punch and switch punishing in Pursuit.
Rotom's STABs are basically independent, Spiritomb's STABs are almost entirely redundant.
Support:
Spiritomb has Nasty Plot and Calm Mind, Rotom has no boosting moves.
Spiritomb has Sleep and Burn, Rotom has Paralysis and Burn.
It remains to be seen whether Spiritomb has Trick.
So basically, expect the physical sets and the Plot Set to be Spiritomb's most used tricks. Rotom just has a lot more it can switch into. Straight out special attacking favors Rotom.