Cayo's Crystal Cave, also known as Mountain Cow Cave, had a great article written about it lately. Great way to put it: Actun Tunichil Muknal is for tourists, Mountain Cow is for adventurers. Both are great ways to spend the day.
"I said it before, but I'll say it again: ATM is for tourists, Crystal is for adventurers. Crystal is different. It was a challenge the memory of which still gets my blood pumping. It is impossible to overstate how incredibly breathtaking it is. It was one of the most exciting and intimate ways I have ever experienced the earth. I will never forget it."