So you got a little glimpse of what it is to be a fisting top. What you're getting off on is that intense pleasure your bottom is experiencing. You did good if you got in to your elbow. That's beautiful.
As a top, I like fisting in bed. It feels more intimate and it's easier to be affectionate with each other. When I top someone in a sling, I always feel a little like a technician. The bottom is perfectly positioned and it's much easier to fist them, but as I said, I feel like a technician.
When the roles are reversed and I'm the bottom, I prefer the sling, because the bottom is perfectly positioned and he can just relax and just enjoy the experience.