First or Third Person?

I think one of the biggest struggles i'm facing right now is: third or first person!? I just really can't decide!! In all of the snippets i've written i've wrote in third person because i always wanted there to be an element of mystery about the protagonist, and about most of the other characters. The only thing is recently i've read some absolute great books in first person and it's made me feel much closer to the characters minds and therefore their thought processes. So now i'm thinking... Do i want to delve deep into the characters minds so it's obvious what they're intentions are, or do i leave that element of mystery, that slight resistance to their intentions? It's a tough one. I know i want to write from different characters perspectives, so perhaps that'd be helped if it were in the first person, so it's clear and not confusing for the reader? (i'm sure writers manage this in third too!) i'm just very torn between the two and i think the only way i'll get to my answer is to write in both and see which one seems to flow better and gives the character the justice they deserve.

Martha