Given that you are going to be on the beach on A.C. I wqould definitely go towards San Ignacio.
In August we did a week of touring around the mainland in a small 4 wheel drive. One of the nights we stayed at Jaguar Paw jungle resort, which is truely an extraordinary experience and right next door to the cave tubing.
The resort is tucked away in a sort of valley and is completely surrounded by close jungle (we found it a little claustrophobic but fine for a couple of days). The rooms are exotic and comfortable, with aircon and the restaurant good. There are virtually no mosquitos because, the owners says, the bats from the nearby caves eat them all! Well it seemed a reasonable explanation to us! The only downside is a 10 mile or so dirt track to get there which in a small car was hard on the behind!
Other optionwe would recommend are to fly to Chan Chich jungle resort - it is probably our favorite of all the places we have been on the mainlandland but we haven't been there for 3 years. Having said that there is no cave tubing near there.