If you're having 5-6 people, go for 2 rooms. Do you want to be in the center of everything or a little more secluded? If you want it to be quieter, look along the southern side of the island (Westin Regina was nice). You can still take a bus to all the entertainment (the public buses are cheap, easy, and safe). Riviera Maya is further south and quieter. Recently I stayed on the north side of the island and liked it. The water was much much calmer, but we were happy with it. I think it was the Grand Oasis Caribbean resort. We got a special deal because they were still undergoing renovations. We booked through
www.bookit.com, and were happy with the results. There's an ad on there now for a few Cancun resorts.
While there, check out Mango Tango's for the dinner show. We also liked Perico's in downtown Cancun. Take a cab there, as I think I remember having to transfer buses and it got confusing. Have the hotel or restaurant call your cab to make sure it's an official one, and negotiate the cab fare before you get in.