Basic vampires in Skyrim are interesting to play as. It makes you pay attention to what you're doing, when you visit cities, keeping fed, how to deal with the odd fire magic user or flame atronach, et cetera. It's only really a useful thing combat-wise under level 40, however, besides the invisibility and calm spells you get. The vampire's drain spell isn't that useful, really.
Vampire lords can be fun to play as, but "blood magic" is nonsense as >>8377 pointed out. More importantly, however, is the fact that the entire Volkihar Clan questline is pretty dull; the entire Dawnguard questline rests upon the player caring about vampire-waifu Serana or wanting the paladin's wetdream Auriel's Bow. Flimsy writing, to be honest.
Lycanthropy in Skyrim is hugely flawed due to the simple fact that you have to be a member of the Circle of the Companions to contract it. Why this was done makes no sense to me, but hey - Bethesda seem to have lost most of their intelligent writers. From a gameplay and potential roleplaying standpoint, being a werewolf is actually quite enjoyable, and is moreso once you have one of the many rings which augment the power. I've never had the scaling issue >>6351 mentioned, even up to level 50+ the sprint attack and power attacks did serious damage to most enemies. Dragons were the only exception, but that isn't surprising.