I've been to both places and I'd never revisit either. Sorry to say, but Mexico is the lesser of two evils. Be very vigilant and mindful of your surroundings regardless of either location. Don't wander beyond the resort areas of either place you visit. If you choose Mexico, Cancun is lovely but stay away from downtown. The beach is very narrow due to hurricane erosion. If you go to Cancun don't miss the Maya ruins!! A nice side trip to Cozuml is nice too. Good luck and enjoy!!