for me, sending out the save the dates ASAP didn't make a difference. and my invites went out in august for a may wedding, so I'm afraid most people have forgotten about it entirely by this point.
we have been planning and waiting and planning and waiting and planning some more for what seems like FOREVER. I'm sure I'll look back and relish this time, but I really wish we'd done it within 9 months of our engagement. I made decisions a year ago and can't remember why and now am re-hashing everything.
I think it's primarily important to choose a resort you like, but you also need to consider other things like the cost and ease of traveling there if you want to be accomodating. don't be scared to just go with what you want, but do be prepared that not everyone will follow you. cancun/cozumel are probably most accessible due to the airport location, followed by the riviera maya and playa, and then it's down to tulum (maybe there's a tulum airport? two hours would be far to travel from cancun, but I know someone who had a larger 65-person wedding there).
you may want to do a website first. looking back, I think that would've been the easiest and most cost-effective way to let people know what we were planning up front. that way they can check out your resort, bookmark it, etc. (ours is here: http://jdandkellyswe....com/index.html - weebly is totally free and super easy to use. I wasn't a fan of the cookie-cutter wedding sites.)
finally, look at the resort reviews on here. it's where most brides start when they're considering where to go, and take a peek inside the different hotel forums and trip advisor review to make sure it sounds like the place for you. that should help you narrow the selection.
best of luck, and congratulations!!