We have been to both very recently - we went to GWL in Ohio this summer and the problem I noticed that was food could be allowed all over the water park vs. in a designated eating area. I have a problem with this - if your walking barefoot all over the place you don't want to step in someones nachos! The GWL in Traverse City, MI is so much nicer, cleaner, newer and bigger. I don't know about your travel from Cananda to there but just a thought - even though the prices are cheaper in Ohio.
The Kalahari rooms were smaller than the GWL but the price was better and the best part was the surfing lessons! Have fun whatever you decide!
BTW I did hear, like someone else posted, the the Wisconson Dells are wonderful!
Good Luck!