Outboard
Not wanting to "rock the boat", I wouldn't recommend anything larger than 9.9 without someone having first hand knowledge from someone who actually HAS something larger mounted on their transom. Don't forget our catalinas don't have the most robust transoms and mounting a 9.9 is a chore to reinforce the transom but seems to be the best "middle" ground as far as having some hp in reserve for beating into wind/waves and achieving hull speed 6.25kts for C27's. I personally have a 14hp inboard (Universal M-18...no this model doesn't represent the hp). I think you would be satisfied with a 9hp model.Don't forget other factors- size, weight, budget, fuel consumption. A 15hp would cost significantly more, weigh more, and the "Catalina squat" when under power would be sigficantly more so.