I've been to Moon Palace twice, and really enjoyed it. We found the beds to be very comfortable - nothing like the rock-hard beds at Mayan Palace, for example. The resort is huge. The food options are terrific - you will not be able to try all of the different restaurants in just one week. I don't know what the food options are at the other resort you mentioned - we prefer AIs that have a lot of choices because limited options become tiresome after a few days.
I have always booked our stays through a Palace owner so am not as familiar with booking through RCI. If you book through RCI I think they give you the option of booking at one price with no resort credits, or for a higher price with resort credits. If you opt for the higher price and get the credits, you will be able to do things like swim with dolphins, massage treatments, ziplining, tours and more and pay only the Mexican govt's 16% tax on the value of the credits. (Example, swim with the dolpins was $129 in resort credits pp and taxeds about $21pp payable at check-out) If you just want to hang out at the resort and eat and drink and take advantage of on-site activities, then you don't need the credits.
As I mentioned, the resort is huge and there is always something to do there, and they have shows in the evening, activities throughout the day, free non-motorized water sports, etc. The beach is not that great - lots of seaweed at times, but they do make an effort to clean it up at least once a day. But if you are at Moon and want to experience a beautiful Cancun beach, they have free daily shuttles up to their sister resorts Sun and Beach Palace, where you can enjoy the beaches and all AI privileges as well.
Booking through a Palace member is the best way to go - I am not a member but I am sure there are some here on TUG. It costs a bit less usually to book through members since you don't have to pay any RCI fees or use any exchanges. But if you go through a member you can only book WITH resort credits. Members get extra privileges and by going as their guests you can enjoy them as well, such as a free mani/pedi and 25-min back massage, two free tours, and more. There's also a nice group on Facebook called Forum for Palace Resorts Members and Friends - you have to request to join, and when you are approved you can read all about the resorts and the site admins can hook you up with trusted members who you can book through.
I am surprised it's only a silver crown. We really, really enjoyed our stays there.