I would say vampire lord form is stronger (that doesn't mean better, I stated I prefer werewolf form), because you can heal a lot easier, you have the insanely overpowered vampiric grip spell, you can avoid any and all melee damage by hovering over water (and strafing to dodge the arrows), can call upon two summons rather easily (the werewolf summons have a cooldown), and all of that can be done from a range.
I vote for Werewolf form, because as Ben has stated, once you get the controls down it is very powerful. My Sorcerer/Werewolf playthrough was extremely OP. I tried the Vampire Lord on one character and became frustrated with how exceptionally wonky the controls are while in VL form. It is powerful, but I am not a fan.
Vampire Lord all the way.