I've been diving Cozumel (and other tropical spots) for 20 years. Did 4 trips to Cozumel last year. I've also surfed for 47 years - that requires wetsuits too.
With the amount of time I've been doing this and the number of times to Cozumel, it's given me an opportunity to make an observation: people react to temperature in substantially different ways.
I know lots and lots of very experienced divers (and people who are employed in the industry) who dive Cozumel with 5mm suits. And they don't get hot in those suits. I also know people who dive 'bare back' in Cozumel. There's no way for us to know which category you'll fit into.
You can spend a month searching for the one best solution and then get to Cozumel and discover that that 'best solution' was really the best solution for someone else - not you.
If it were me, I'd get the 5/4 - because I know what I need.
But for you, since I don't really know what you need, I'd recommend a contingency plan: get the 3/2 in case you're one of those that does not need a lot of thermal protection and rethink the vest thing as an insurance policy. As you know, there are lots of different types of neoprene vests. There's also some nice Polartech vests made by O'neil (they're marketed to the surf community - not as restrictive as neoprene and pretty effective)
I usually do not buy gear in Cozumel but they do have shops there. Prices and selection tend to be better in the states. If you get there and need to buy some gear, there's a nice shop on the corner of Av 5 Sur and Rosado Salas. There are other shops, but this one has a good location and pretty good inventory. In the evening, Edwardo is usually there - great guy, easy going, not pushy, works as an instructor during the day so he knows his stuff.
I'm going back next month. I can hardly wait.
Have fun.