Forget about the water slides at these two resorts. There are much greater differences to compare. The Grand Floridian is Disney's "flagship" hotel, meaning they consider it to be their best. It is very luxurious, with elegant Victorian architecture, a live orchestra playing in the lobby, etc. The Contemporary is 30 years old, was futuristic looking 30 years ago, but now appears very dated. The monorail does go through the lobby, which is cool, but in terms of decor, the Contemporary is extremely plain when compared to the Grand Floridian.
All this being said, they are both deluxe resorts and while there are many that say the GF is best, there are also those who swear by the Contemporary.
Check out some pictures of both places, and read about them before making your decision. You may love the Contemporary, but I could see some folks looking across the lagoon to the GF, saying to themselves, why am I not staying there?!?!?!