Aventura has a man-made beach that fronts a man-made lagoon. That part of the coast is rocky so there is not a natural beach. If you're looking to swim in the ocean and experiences waves, Aventura won't give you that. Xpu-ha Palace has a much better beach (beautiful crescent beach), but it is a very different resort than Aventura - much more adventurous. Think of it as rustic luxury. Personally I far prefer Xpu-ha, but it's not for everyone. There's lots of walking as it was built on an eco-park and is spread out among a large chunk of the jungle. Xpu-ha has a great beach, two nice pools, fresh and salt water snorkeling, kayaking and fascinating jungle with animal habitats.
If you're looking for a typical beach hotel, Xpu-ha's not that. But if you want a really cool and different experience, you can't beat Xpu-ha. I think it's hands-down the best Palace resort. A little bit of everything.
Here are some photos from our trip last year to Xpu-ha. We went back again this year and loved it!
http://community.webshots.com/album/76674802FtReaA