I've never been to Moon Palace but I have seen their reviews plenty of time and it just never grabbed me.
We stayed in "The Royal in Cancun" back in 2009. All inclusive hotel that is for adults only. Awesome food, 6 or 7 restaurants and all are amazing. This is the hotel that is truly all inclusive. The rooms are really nice with jacuzzi inside, staff are excellent but then again, I think most staffs in Cancun are great in general. Alcohol downstairs, in the pool, beach, and in your rooms, ALL FREE! Lots of ammenities, things to do at the hotel and they have games/sports throughout the day.
I highly recommend this one especially if this is your first time!!
We also went to ME by Melia earlier this year and it was fine also but food is not as good, less restaurants, hotel is smaller and therefore a lot less ammenities. It was about $300-400 cheaper than The Royal but I would've paid more in a heart beat if I knew about it.
Next year we are planning to head back out there and stay at the Royal again. That place left a really good taste in our mouth that we just have to go back.
Advise: Arrange a shuttle service from the airport to your hotel. Disregard anybody else once landed in the airport, people will hound you for time share, etc. Go straight outside and find your shuttle service whatever company name that be. Your shuttle company's name will be all over on their employees t-shirt, so only talk to those people.
Say no to "tours" in your hotel, timeshare. I spent half a day with this agent telling me how good this deal was.. blah blah blah. The only good news was that I got $100 credit, not bad but it may not be worth your time. It might be your thing, it wasn't for me but I thought I should share that to you as well.
Have fun. People go to Hawaii, we go to Cancun. I love Cancun.
Nim