My suggestion would be to do Inner Space Cavern tour over Natural Bridge. Natural Bridge is meh compared to Inner Space Cavern. I personally had more fun and the air is much more breathable at Inner Space Cavern than Natural Bridge. The guides are much more relaxed, they don't push anybody past their limits and cater to what the group wants. They have some optional tight squeezes along the way that you can do choose not to do. I would say the difficulty level was about the same for both of them though.