We stayed in RPLA last July and absolutely loved it. The main difference that we could see was Rui Cancun was a much bigger hotel and has a more party atmosphere. Where the Palace had what they call "a party pool" which ment that it was this pool that the games took place in the afternoon. The palace also has in infinite pool right on the beach. The Cancun has a disco in the hotel. Your are right as a palace guest you can visit any RIU hotel but only Palace guests can go into Palaces.
I don't think you will be disappointed in which ever you choose, it is a matter of choice and what kind of holiday you want. If you like a more laid back hotel, then stay in the palace and pop next door to the Cancun - best of both then!! When we were there we were a group of 8 and the only other british guest were all with thomson (the rep was first class by the way) I think our first week there were 2 other british parties and the second week 3.
Have a fantastic holiday.

Thanks for your reply, whilst I am more inclined to go for the luxury and the quiet relaxing side of holidays, the Palace does sound more appealing to me.
But, there will be quite a few of us plus we will be celebrating my daughters and maybe another couples daughters & sons 18th so as a family and as a group maybe we will be better off with the Cancun.

We will miss the US Spring break, so hopefully it will only be slightly lively rather than nuts
The Palace is working out over a £1000 more expensive for us as a family, which as we are going for a special occasion (Its my 40th too) I don't mind the extra cost but by all accounts I think we are going to be better in the Cancun.
Thanks for getting back to me. Only 72 weeks to go

I know, its far to early, but we need to decide on the hotel before the brochures come out in April for next year in order to grab the best early booking discounts.

It is so difficult to choose especially when it is such a special occasion and others are involved. My two were 17 and 21 and my neice 15 when we stayed in the RPLA, they did go next door but felt we had definately made the right choice with the Palace.
Hope you get a good deal when booking.