We have been to Mexico twice end of Aug/Sept (2012/13) First year we had one shower during the day which lasted about 20 min . The following year we went over the exact same dates and we encountered a lot of rain over the last 4/5 days. As for the heat, I didn't encounter any problem as there always tended to be a nice breeze, although on occasions the evenings could be pretty humid. We are returning again next year, but June this time.
We went to Aruba 4 years ago as it is outside the hurricane zone, and I really found the heat very intense, so much so, I spent most of the day under the parasol! Also, the humidity in the evenings was a killer lol!