They are two completely different boats:

Here's the quick and dirty:
F18 - low tech construction, lots of carbon limitations - platform weighs 397 lbs

F18HT - utilizes carbon in the beams, hulls, and mast - platform weighs 265 lbs

F18 - sloop rig with spin, limitations on sail cloth materials

F18HT - uni rig with spin, lots of exotic sail material choices - cuben fibre, carbon/mylar, etc.

F18 - fleets are popping up everywhere in the US - probably growing faster than any other class

F18HT - a couple of localized fleets, and the remainder are spread out all over

Opinion - I've owned and sailed both platforms extensively (literally 1,000's of miles). The 18HT is a neat boat for lake sailing, the F18 is better in big air/big waves. If you want a boat that you can improvise/engineer/modify go the 18HT route. If you want to race in one of the largest one-design fleets go F18. If you want to do both, buy an F18 and an A-Class Cat - then you'll be in heaven...