Where's the best spot to vacation in Mexico? Cabo, Cancun, or Puerto Vallarta?

I'm planning my first vacation to Mexico, and I'm not sure where the best spot to go is. I live in Las Vegas, so I don't need a 24/7 party scene. I'd like to go somewhere that is a good mix of relaxing and partying though. My husband and I are in our mid-20's so we don't want to be surrounded by high school kids on spring break or a ton of kids, but we also don't want to be the only non-senior citizens there. Also I'll be going in mid-July if that makes any difference. Thanks!

I've heard Cancun is a major college / high school kid party destination so I've avoided Cancun. I don't know much about Cabo. We have been to Puerto Vallarta (Mayan Palace Puerto Vallarta) and LOVED IT! We are in our early 30s with one child. The weather was awesome (we went in May) and it was fairly close to the pier (restaurants, a church, shops) and they usually have some sort of festival going on. We went to Vallarta Adventures and I swam with the dolphins...amazing! We also took a boat ride to Yelapa and had lunch on the beach and then rode horses (well, that's what they called them...) up the mountain. Very relaxing and fun! I'd definitely recommend PV.

I am a mexican and I've had very bad experiences with Cancun and obnoxious spring breakers (all american kids), it is a real shame to find this because Cancun is so very beautiful, the historic heritage, the water quality (at the sea), the soft sand and the rich fauna are so worth it, this is why -thank heaven- somebody thought of us and moved a little bit west to make Playa del Carmen the perfect touristic locations for us all who want that relaxing and partying mixture.

This is very near Cancun and it's as beautiful.
Do not forget to the historic heritage too!
Chichen-Itza, Tulum and Xel-Ha are a must

I totally recommend it, you and your husband will absolutely love it.

FIRST OF ALL, ALL THREE RESORTS YOU MENTION GET SPRINGBREAKERS, THE TRHEE RESORTS ARE GREAT, YOU CAN GO TO A VERY EXPENSIVE HOTEL, YOU WONT SEE SPRINGBREAKERS THERE, IF I WHERE YOU I WOULD WAIT, AFTER THEY ARE GONE, YOU WOULD BE STUCK IN THE HOTEL, AND IN THE EVENING THATS WHEN THE TROUBLE STARTS