At 230lbs, you'll be VERY powered up in anything over 8 knots.

Personally, I feel that you should probably stick with the F18. If you can't get that thing powered up at 230 pounds then simply upgrading to the N20 isn't going to solve that problem. There are plenty of SoCal F18 sailors that get their boats plenty powered up with more than 230lbs of crew weight on board.