- On the 16 the tramp is empty, while the 18 has dagger boards which stick up a little and jib blocks and tracks in the middle of the hulls.
- The 18's dagger boards must be raised for landing.
- The 18 will handle 4 adults, but I'd be reluctant to put them on a 16.
- Rigging a 16 is a little easier.
- The 18 may have deck ports and hatch bags for storing stuff.
- Waves flow through mesh tramps way more than solid vinyl.
- 16's pitchpole easier than 18's, but as you get better then you learn how to avoid this.
- The 18 has a smoother, more stable ride than the 16.
- The 18 racing class is dwindling since the boat is no longer in production. However, Hobie promises to keep parts available for a long time.
- Delam? Duct tape? Hmmm. . .
I may be overly cautious, but the first thing I did when buying both my boats was to replace the standing rigging. It's cheap insurance against a dismast.
Both boats are OK for a beginner, as long as you put the time and effort to learn good technique which will make the whole experience more enjoyable.
Also, read up on this site for more buying tips:
http://www.sailingproshop.com/catamaran.asp
Welcome aboard!