well ts not like they want to do it. *cough cough* rosalie. they didnt choose it. they have to live with it. but then sometimes someone comes along who loves them enough to look beyond all that and be there for them.
a werewolf, well twilight ones, cant choose either. its in their genes. the cullens choose to try to make the best of it by drinking animal blood so they attempt to make it good. so some vampires choose not to change humans. werewolves change whether they like it or not. its chosen for them.
Vampires creep me out, and there is no way I could be around one as human. So I would have to become a vampire for things to work out, however the immortality does not appeal to me so I do not really like that option.
So that would leave me with werewolves being my preference. I find the whole wolf pack fascinating. So I would much rather be around them than vampires, but I am not really sure that I would want to BE a werewolf. I really feel for Leah being the only girl, having to hear their thoughts all the time, that would be really annoying. But the best feature is that there is the option to stop once you gain control and being able to live a normal life.