I currently have a Nacra 5.2 modified for single-hand with a Pentax sail.
In the past I have had: H14, H14T, H18Mag, H17.
Personal opinion:
H14/H14T - Small, not much boyancy, fun boat for someone by themselves a lot. Otherwise No thanks... slow.
H18Mag - A war-horse. Not for single-handers. A hell of a lot of fun with 2-3 people in a good breeze. Built extremely tough, but also over 400lb.
H17 - Fun boat for 1 or 2 people. Not a light wind boat. The old mylar sails were terrible. The build quality is the flimsiest of all Hobies. For a single-hander it is a fun boat though. The centerboard design is down right moronic. Easy to move around on your own.
Nacra 5.2 - Solid. Under 300lb. Parts are generally cheaper in comparison to Hobie including sails HT or otherwise. I have never been a huge fan of the Nacra rudder system, but I'd rather replace ropes instead of Hobie cams. Generally fast boat before HT sails. After HT sails, wow. Pre-1983 N5.2's did not have foam sandwich hulls. Just straight fiberglass.
Basically: If you want to race one-design, buy an H16. If not, I recommend Nacra. From what I have seen Nacra generally makes a higher quality boat. Nacra has more support overseas than they do in the states. Parts will always be around and there are a ton of sailmakers. You aren't locked into buying MFG only parts.