Aquatica=nicer,newer,Roas Rapids is the fast lazy river which is the best attraction in the park,2 kids areas one if which is fun for even adults,6 major slides 2 of which are multi person raft rides,the slides are pretty thrilling overall but nothing extreme.Best dining and shopping of any waterpark in Orlando,two good wave pools,overall a very clean and fun waterpark.
Wet n Wild=Older but still maintained very well for it's age.A good amount of multi person slides,so if you prefer multi person slides this may suit you.Has more slides that could be considered thrilling as well as unique with slides like the Brain Wash or the one that has disco music inside.Average wave pool and below average small regular lazy river.
Basically if you want the nicer waterpark with more open spaces to relax in sandy areas with just an overall nice atmosphere and good slides,lazy rivers,food and dining go with Aquatica.If you don't care about a lot of concrete,less space or sandy areas or average wave pools or lazy rivers but more unique and multi person slides go with Wet n Wild.Wet n Wild also has a lake that used to have free knee skiing back in the day but I believe they now charge,I did it when it used to be free don't think I would do it if I had to pay though.