Not having sailed an F18, I can only vouch for the N20, which I find to be the best for heavier crews. I am 180, my crew is 190, and we were on the lighter side of some of the teams we raced (and occasionally lost to!)

That being said, there is a rumor that it takes a bit more "beef" to handle the cordage on the N20 (spin sheet, main, etc.) but in my conversations with those that have raced both F18 and N20, I think that is just a rumor.

For example, I believe the spin halyards are of similar diameter, and yanking the spin up/down on both boats can get hard on the crew. Spin sheet loads aren't much different, unless you're sailing an F18 with the small sail plan (which you wouldn't with your combined crew weight).

The F18 fleet is growing on the buoy circuit, but I haven't seen tons of them in distance racing. There are a few F18 owners who used to (or still do) own N20s, and are just chasing the rock stars as they move from fleet to fleet. Most of them were sailing the 5.5s before the N20 if memory serves. It's kind of a cyclical thing.

Whichever boat you decide on, know that they are built strongly, well laid out, and parts / service are readily available. You will have plenty of fleet racing in either group (if you are okay with driving to regattas), and there are top notch sailors all around.