Would deffo say the plus, the food was about 370 Peso I think, plus drinks. You get 2 drinks and don't pay for the food. You also get free lockers and scuba gear.
Definitely go "plus", we visited Xcaret on Sunday and the food was expensive! More than 300 pesos for the buffets, can of Coca Cola 20 pesos. With the plus you do get towels, lockers and your snorkel gear included whereas with the basic you pay extra for those things.