While there are a lot of factors to be considered, in the end it is indeed math: mana cost to do damage, as Dragoth said. So it shouldn't be too hard to work something out for the necro pets, IMHO.
I have a very limited experience playing a necro (34ne in s3, to 2x40 as secondary on s4), but I do agree that it's more efficient to use animates for exp runs, rather than summons, because:
- wights have slightly better hitroll
- wights have more hps
- animates last twice as longer, than summons
- you can use your xp mob corpses to animate, so you only need to use summns for the first few exp mobs (assuming you are killing at least wightable mobs for xp)
- other animates have useful procs (vampires can suck blood from mobs to last longer)
I also have to note that for some exp kills it's possible to wall pets, rather than sanc, which makes walling even wraiths useful. Also take into account the actual mana cost of the summon undead spell. Flesh golem is nice as well.
Obviously, I didn't test much pet points or pet procs beyond wraiths or liches

I also suspect that Dragoth's preference is partly influenced that he has high mana regen and can afford summonning and sacrificing undeads instead of animating them and lighting them to save mana (keeping undeads alive = saving mana).
It's weird that none of you has mentioned a xeg-yi, who heals pets: does it mitigate any downfalls of any pets or it's just useless, since it takes up a good portion of pet points?
Thanks.
P.S. I think Dragoth addressed his posts to imms, who don't play, rather than more experienced necros, Thraxas. I might as well can be mistaken.