Probably depends on what interests you most and what strengths you have - omaha will certainly be the easiest step in mechanical terms, just remember that you have to use exactly two cards from your hand: no more, no less.
On the limit or no limit question... the game pretty much decides it for you. Certainly if you're playing stud games, they'll pretty much always be played in a limit format. Omaha is usually played either pot limit or no limit, unless you're in a HORSE game.
Of the stud games, I found Razz the easiest to pick up, but it's probably the most frustrating as well