Here's what I find, the VL abilities only truly benefit a character invested in magicka, so its slanted heavily towards a mage character. However the basic human form of the vampire has abilities that greatly benefit an assassin especially one who uses illusion. You may want to go Vampire Lord simply for the basic abilities and ignore the transformation.
The only real requirement for a werewolf is maybe alteration, they regenerate stanima quickly enough that heavy investing isn't necessary(it helps but isn't a requirement like magicka is for VL abilities). The reason a werewolf wants alteration is for perks like atronach or magic resistance to buff up his magic defense and possbile for the armor spells, A vampire lord has the same concern. As both transformations don't let you keep your customized equipment magical defense is a big issue.(and your base Breton/Nord/Dunmer resistances won't work after you transform). Agent of Mara+Magic Resistancex3+Lord Stone=70% magic resistance. A great benefit to either vampire lords or werewolves.
So again my advice to you is go vampire lord but don't bother much with the transformation unless you have a lot of investment in magicka(i'm guessing your assassin used cost reducers for illusion).
I contracted lycanthrapy on my mage character to get rid of this stupid magic glitch that makes people think my undead thralls are summoned all the time when they arn't.
Going to Jail or casting a cloak spell also removes those active effects for future reference.