We enjoyed both but I would recommend splitting them up into different trips.
Xelha is a protected reserve offering snorkling, tubing and a dolphin experience. It is an All-Inclusive park that includes your snorkling gear ($20 deposit) and several restaurants where you can just sit, eat and drink if you prefer. The dolphin experience does have a charge but since we didn't have the time to do it I can't remember the cost.
We had a good time however since it was a combined tour we didn't get to spend enough time at Xelha. If we had the chance to do it again we would have spent the entire day their.
I'm a history buff, especially the anicient tribes of the america's. Walking through Tulum has great, especially with the excellent guide we had. I got to spend some time discussing the ruins with him after he release our group to roam around and was throughly impressed with his knowledge. Though not nearly as majestic as Chitzen Itza, Tulum is special for it's location and importance in the trade rutes of the Mayans.
If you want to do this tour I would suggest combining the day with a trip to Coba. Especially after I heard that you can still climb the pyramid there.