Tulum is the Mayan ruins. You will want to spend the $1.50 for the trolly ride. You cannot climb the ruins, and a number of the more interesting things are blocked off. However, it's a nice site and is well worth the trip. You have only a little time at the end to explore by yourself. However, they are genuine ruins and well worth the trip.
XelHa is a well developed area of stores, beach, dolphin encounters, nature parks, and so on. There are ruins that may, perhsps, be seen, but they're across the street. Think of this as being a resort a bit more like Disney world than anything else. Mayan stuff, you won't see (we didn't). There are a few things to be seen, but not on the order of the ruins at Tulum.
If you want ruins, go to Tulum. If you want beach, dolphin encounters, parrots, stores, and probably restaurants, then XelHa is the place for you.
We wanted more time at Tulum, and less at Xelha....
If you're going in the summer, take icewater, sip it lightly, and a hat. You did not need bug repellant in July, but it was hot....