Depending on unit size and when you want to go it *may* (and I stress *may*) be "cheaper" to use HGVC via RCI to get the Grand Mayan in Riviera Maya.
I think the HGVC option is via SFX and it's a bit "pricey" vs. the 3400/4800 RCI will charge. Assuming you can get it.
But the Riviera Maya resort is *almost* half-way between Cancun and Play del Carmen, so if you want to go to either place it's a bit easier.
We went from Grand Mayana to Xcaret via the "bus" option (we bought the tickets with the all-day/night show and transportation from the resort). But we took a cab on the way back (that was a bit um, interesting).