[Kobal2]

The main difference lies between soulless and longdead.
Priests or spells that reanimate longdead (skeletons) always have the same effect, no matter where you cast them. That's because they reanimate dead people who have died a long time ago, and there's never any shortage of those.
Priests or spells that reanimate soulless (zombies) are different, because they reanimate dead people who still have some flesh on them. Thus, they're a little bit wasteful if there are no fresh corpses around (i.e. your mage reanimates fewer total people/hitpoints than he would have reanimating longdead), but are better when done in provinces that have corpses in them (in which case you'll reanimate ~ the same number of undead, except soulless are generally much better than longdead man-for-man).

The same is true for battles BTW: in the first turns of a battle animating skeletons will get you better results. A few turns in when some people have died on either side of the battle animating zombies becomes more efficient.

Finally, there's the Raven Feast spell, which turns Air gems into Death gems based on the corpse count of the province it's aimed at.