I go to both, but if the price were the same, I would choose GWL. I think GWL is better for smaller kids (under 42") as there are more areas. Fallsview allows 42" alone on their tube rides but at GWL they need to ride with another person. Overall, I think Fallsview has more slides for the adults (2 non-tubes and 2 mats), but it doesn't have the lazy river.
As dragnbiz said, the lockers are a plus for GWL and since I usually stay at the Hampton (for the lowest rates and breakfast), we need to walk through the casino parking lot.
Fallsview has the benefit that you can get food from any restaurant you want, at GWL, I find we eat onsite (not required, but who wants to load into a car and drive after swimming all day) and it is poor and expensive (esp. dinner).