It's personal choice really. For me it would be Blizzard Lagoon, or would that be Typhoon Beach[msnwink]I love them both.
Personally I think Typhoon Lagoon offers a tad more in the way of shade especially when walking between the rides, although there are plenty of shady spots at Blizzard beach as well.
The floor does get very hot especially Blizzard Beach, although they spray the paths to keep them cool. Flip flops are a good idea for walking around although you may be asked to remove them and hang on to them on the slides. I have seen people walking round in those little rubber shoes which they seem to be able to wear on the slides also.
Whatever you choose have a great time!