Hi Declan,
Welcome to the board. I have been to Mexico in early, mid and late October and in the early to mi periods there is a lot of rain. And even if it doesn't rain, the winds soemtimes make the seas rough and the ports close. If you have your heart set on that date I would suggest having a few backup plans. For example, if it is a really crappy day outside, plan a dive to the cenotes. Who cares if its raining or the seas are rough. That won't affect thsoe dives!
As for going to Belize, we did that trip about a month ago. What we did was spend the day diving all day, went to dinner and out on the town a bit. Then caught a midnight bus from Cancun (downtown) to Chetumal. It was a very clean,non-stop bus with plenty of leg room and we used the time to catch about 4-5 hours sleep. The bus cost about $18 one way which was a pretty good deal. So when we arrived we were fresh and ready to enjoy a full day. Then we got a ride from Chetumal to Corozal Belize (about 20 minutes). In Corozal we took a small plane to Ambergris Caye ($35) and then a water taxi to our resort (Gaz Cooper's/Playa Blanca Beach Resort). The trip from Mexico to Belize was a lot smoother than I had thought but a lot of that was due to Gaz's arranging an experienced guide to assist us through customs and immigration and who knows the ropes. It went very smooth. And by going to Belize we got to check out some dive sites we have never been to before.
I know this is a Mexico thread and I sort of got sidetracked here but I guess what I was getting at was it isn't that difficult to do both.
I can recommend a few dive ops in Cancun, Puerto Morelos and Tulum if you like.