I usually like to have some way of generating a corpse counter first turn when playing seamus.
The reward of a cheap belle or doxy is usually going to be worth the investment.
Can be done with Mortimer, a dog, or any henchman with corpse bloat. I sometimes put corpse bloat on seamus, although you can't drop and summon in the same turn using this trick.
I base my decision on which to go for on the strat/scheme pool, and what else I will use the model for during the game.