If you were having to choose between the locations (Cancun or Isla Mujeres) then I would say it's a

Friend ("of a certain age") and I plan 5 nite stay in late Feb/early March and would appreciate recommendations for a good **** resort in either. We don't want to do water sports, altho a pool/ocean front is a must. Need great restaurants, interesting bar, lovely staff, comfortable room, not isolated from off-site activity, but not crowded with spring-breakers.
If you were having to choose between the locations (Cancun or Isla Mujeres) then I would say it's a no brainer... Isla Mujeres. This said, you seem to be more interested in the resort than the actual location. As much as I hate to say it, Cancun would have to be your choice as the selection of resorts there is almost endless... you decide !
Isla Mujeres hands down, was there in May for 3 nights. Check out Ixchel condos or you might like Privileges Alux or Villa Rolandi (this one is south of the main beach area and town so you would need to take taxis or rent a golf cart.) I was lucky enough to stay here 1 night with a friend and the service and food is first rate. For such a small island (5 miles long) there are over 130 restaurants. We took the private boat back to a dock in the hotel zone and en route to pick up our rental car we saw maybe 2/3rds of the hotels, OMG never saw so many high rises in my life. Lots of people like it though.
Based on your comments, I'm not sure Isla Mujeres is the right recommendation. Although the main beach area is lined with restaurants, there is really nothing much else to do there. The island is quite small. You can rent a golf cart and drive around the whole thing in about 20 minutes. If you are looking to just relax and chill for 5 days, then yes it would be a great place to stay, but when you said "not isolated from off-site activity" then I wouldn't think this is the place as there are really no activities per say.
The Playa del Carmen area is a better option. You will definitely cheapest tickets plane on the web be closer to off site activities. From the Cancun airport it's about 45 minutes to an hour depending on the hotel you chose. Most hotel/air packages offer a shuttle cheapest tickets plane on the web to your hotel for an extra charge. If you go with an adult only/couple only resort, you will not have to worry about spring breakers. However, the times we have gone in April, we had more issues with the thousands of kids than any Spring Breaker.
The taxi from the airport to Playa Carmen will cost you about 65.00 US which takes about 40 minutes (by car). If you take the bus (ADO) it will cost 120 pesos which is about 11.00US and will take an hour. You have mentioned a 5 nite stay, which is not a whole lot of time. Although I am not an AI kinda guy I have stayed in Cancun on 2 different occasions in my younger years. I cannot recommend any AI hotels but I do know that there are some real beautiful ones located right along the fantastic beach there and the food (as you know) in Mexico is really hard to beat. Make sure your hotel is on the ocean side and the the lagoon side. If this is your first time in Cancun, I am convinced that (with the right hotel)you will relish your every moment.
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My vote goes as well for Playa del Carmen, but choosing between Cancun and Isla Mujeres, is a very dispair choice, I mean, I really cheapest tickets plane on the web like Isla Mujeres, cheapest tickets plane on the web but it is very calm, is a place for relaxing. While Cancun is a place for party, or doing outside activities like tours to Chichen cheapest tickets plane on the web Itza, the Jungle Tour, etc. Playa is more balanced, and the Quinta Avenida is a charming street. And if you like a plce with action and not isolated from off-site activity, Playa del Carmen would be the best choice. You could stay at The Royal Playa del Carmen, I think it changed its name to Hyatt, but the resort is just a pair of streeets away from the Quinta Avenida. Enjoy your travel.