After sailing an H16 for years, I crewed a race for a guy with an 18 a couple weeks back. Prior to that, I had always kinda wanted to upgrade to an 18. Putting more than 2 people on a 16 is kind of a non-starter, and while I don't want to go 4-up very often, it is a good capability to have when you want to share your love of sailing with people in from out of town or whatever. After crewing that 18, I no longer see the 18 as my "next step." My primary complaint is that the trampoline area is very "busy" compared to the 16. The jib travelers (a nigh to useless adjustment as far as I can tell, given where/how they're mounted) and associated blocks are in prime "banging-your-knees/shins-into-them" position, and the issue of "tramp spaghetti" seemed WAY worse on the 18 than on the 16, although that could just be the fact that it's different, and I wasn't used to it like I am used to the 16. My secondary, somewhat related, complaint would be that it was a pain to handle on the downwind leg. Because the jib trim rigging is in such an odd (to us, anyway) place/orientation, the crew essentially has to perch way up at the front of the wing and hold the jib clew in his hand when trying to go dead down. It's not something I'd bother doing if I were sailing purely recreationally, but it sorta reinforces my belief that the jib trim rigging is not the greatest on the 18. The wings are super nice to have. It was a fast boat, and felt considerably more "stable" (for lack of a better word) than my H16, but if I were going to get something bigger than my 16, I'd look for a 21SE I think.
Sailing dead downwind is not the way to go on a H16 or H18 (or to my knowledge, any cat). You need the wind to blow over both sides of the sails in order to generate lift. If you want to optimize downwind speed, you need to tack somewhere in the neighborhood of 90 degrees apparent wind. I personally feel the jib system on the H18 is easier to manage than the jib system on the H16. That said, I agree the H18 tramp can get messy, especially when you're sailing with more than one crew.
As for the notion the H18 is "more complicated" or more of a handful on the water than the H16, I disagree. Yes, the 18 has daggerboards which you have to raise/lower as you approach/leave the shore, but this is hardly complicated (and I solo my 18 most of the time). And on the flip side, the H18 is an easier boat to tack. Assuming you have half-way decent wind, the H18 skipper really has to screw up to get his boat stuck in irons.