Cancun is more for relax while Puerto Rico offers more

diversified activities like rainforest, beach, old town, etc.

I love both and it really depends on your own needs for this trip. My LG and I take 2 vacations each year, and one is activity/culture based and the other one is definitely pure relaxing, nothing more than eat, drink and lay on the beach.

Also, in terms of cost, Puerto Rico costs more than Cancun (all inclusive resorts are really budget friendly)

Have fun!