If you're not going to ride the road bike, then definitely go Fat Tire. Otherwise, I'd do novice. Starting your first OWS with the Clydes/Athenas could be an adventure.
If you want to try some OWS practice, there's a small group of us who meet a couple times per month at Anclote River Park to practice, including swimming into/over each other (occasionally, we even do it on purpose). Tentative next swim there is 6/27 ~7:30AM
PS - I'll be slogging my way through the Xterra on Sunday.