I haven't been to GWL so maybe I shouldn't respond, but nevertheless I am going to respond
I think Massanutten is better if you want to do other things in addition to the water park such as skiing. If you're only going to the waterpark, I would do GWL.
I have been to Massanutten and Camelback (for both skiing and water park), and we are going to Camelback again because I preferred the water park being IN the hotel. Of course the location of Massanutten is better for us, and we did like it. I would go back again. I do want to go to GWL.