I have stayed at both and have my own bias opinion on them. I like the Marriott much better, was a bit closer to the park and had better service. Also, like the pool/spa area better. The Hyatt does have a better lobby though and cheaper parking. Marriott is 12 a day and Hyatt is 10 if I remember correctly. We went to the Hyatt 2 weeks ago and the water pressure was horrible and never seemed to get really hot. The Marriott always gets good reviews with us. Their shuttle doens't stop at any other hotels like the Hyatt does. Even with that said, I would be happy to get either with Priceline since you are getting such a deal. If I were to pay, I'd chose the Marriott, but if the price difference were huge I'd go with the Hyatt, I am also a on a budget!
Hope this helps.
Jenn