Wooooooooo for the news, didn't know they were in
Anyway on the choice... both, across two characters. Or possibly across one, I imagine contracting one disease doesn't prevent you from contracting the other once you've cured the first.
But what will my first character's disease be... hmmm, not sure. Werewolves are "new", but I really loved being a vampire in Oblivion, it was both awesome and very useful. I'll have to see what stats you get from being one, if a werewolf is more brute-strength oriented (I imagine vampires are more attuned with magic), I might have it on my first character as the first character will be a ranged assassin (not sure if his bonuses will align for a bow and arrow though). Will have to see in game though, I'm not sure how it'll fit in with my RP, I love vampires more than werewolves overall, but definitely wanna try out both.