Wellllll..................... Everything is relevant.
Noise: All "Sport" boats are loud. The majority of the noise is just wind. So, even a quiet boat, at 35 mph will be loud to the point where you can't have a conversation. If you want to cruse around, an talk with your buddies... buy a Pontoon, with a 9hp 4-stroke. (won't go more than 8 mph)
Water: It's an 18' sport boat, so the bow isn't very tall. SO... yes... you will get splashed if you are driving aggressively, or try to go out in rough water. I've been out on busy weekends with my Islandia and have gotten splashed. So, as you may imagine... it's not well suited for rough coastal waters. (although the 20' will be a little better)
With all that said... it's a sport boat. SO, if that's the kind of boat you want... then you have to accept a little noise, or water. But then again... that's the point of the boat. If you want quiet, and no noise... then you won't be pulling a tube/skier, or having much "fun."
Rotax Vs. Merc: That's a tough one. The Merc is not cheap when there are problems... but then again... with the Rotax boat... you have 2 engines. Not to mention, things like the computer on the Rotax boat are starting to get dang near impossible to find, if you burn one out. (and it's not cheap either) Also... with the Rotax boat... generally... when something happens to a drive part... it will happen on both sides.
Even though the parts may be a little more expensive on the Merc boat... you can still buy them at any Merc dealer. Most of the Rotax parts are discontinued. (but are still out there)
I use to be say to stay away from the Merc boats because of the price tag of the parts. BUT, they are very reliable, and I am happy with my 240 Sportjet in my Islandia.
So....... I guess I would say.... look at the price tag, and see if ether boat has had it's engine(s) replaced yet. Because, if they are the originals... they are at the end of life, and there will be $$$ in your near future.