Cancun or PV?
 

Which destination is right for us? We are three women in our late twenties, looking for relaxation and fun. We'd like to stay in a nice resort-type hotel, preferably with a spa. We will mostly be swimming (a swimming beach is essential) and doing other water activities. We'd like to do for 1 week next summer, and of the 7 days will prob. spend 1 to 2 days out of the resort exploring. Any suggestions?

If a swimming beach is essential, Cancun. I personally like PV better, but I normally prefer to be poolside rather than the beach so it's not as much of a concern. With your priorities, pick Cancun or Play Del Carmen.

I am a big fan of Puerto Vallarta, but for what you describe, sounds more like Cancun-style. More white sand, turquoise water on the Caribbean side. Beaches in PV can be a bit rough and rocky. Summer is very HOT in Mexico, you realize?

PV has a charming authentic downtown area, but there are more high-end spa-style resorts on the Cancun strip.

I love both PV and Cancun, but as others suggested, Cancun has a much better beach for swimming. If you're not much into the party scene, you may look further south to the Mayan Riviera such as Playa del Carmen. There are some nice properties down there.

I love the JW Marriott in Cancun. Great spa, great Everything!

Will Cancun really be so much of a party scene in the summer? I am hoping to avoid that. We are looking into Le Meridian.

I think LeMeridien would be beautiful, and it is in the nicest part of the hotel zone; there won't be a party scene at the better hotels.
But it's also very easy to hop on the bus to go to the bars and restaurants.