Mexico. A top destination for American tourists and possibly the best place to have spring brake. I've traveled throughout Mexico and have had a spring brake in Cancun as well as a bit less traditional Tijuana. When I went to Cancun, I was there purely to have as much alcohol as I could. It was a blast. Tijuana, on the other hand, was more of a side step to my spring brake. I was on a road trip in California and drove down to the Baja peninsula and spent a few days in Tijuana.
Cancun
We have all heard the crazy stories from people that went to Cancun. The party doesn't stop. I've been there a few times and every time was more fun than the last time. You find more cool things to do every time. I think you can go there every year in your life and still have more things to do. As much as it is catered towards the younger crowd in certain months of the year, there are lots of things to do for pretty much any range group. You can get a lot of experiences there. You can go to the bars or just relax by the pool. You can also go and check out Chichen Itza and check out the Mayan ruins or go take a boat trip in the lagoon. Fun for the hole family... or leave the wife and kids behind and do some things you will regret when you come back. Either way you'll have a fun time.
Tijuana
Tijuana is another great city in Mexico that is fun to visit. For people living in California it's only a short drive away and for anyone else it's well worth the trip. You get to see how people live outside of the US and really see the true culture. The street food there is better than anything you can get at a restaurant in the US. The streets are busy with people and there is always something going on in the city. Also if you go a bit further down you can have an amazing drive in the mountainous region and end up at a little beach to have a few beers and dip your toes in the water, though I must warn you the water is pretty chilly. When you are on the Baja peninsula you feel a sense of freedom. You have so much exploring to do when you are down there. If you drove there, the way back across the border is just very interesting, it's almost like a roadside mall with people selling all kinds of cool things from souvenirs to t-shirts.
The Verdict
I would say do both if you get the chance but as far as vacationing goes, Cancun is a winner by far. You can relax there and that's what a vacation is all about. Don't get me wrong, Tijuana is great in its own way but it's a very busy vacation. Cancun is laziness to the extreme, and everyone needs a bit of that from time to time.