Which variation is best? They're both an amazing pec, tricep and shoulder builder (shoulder health issues notwithstanding). I personally have been gravitating toward ring dips, however. They're more difficult, as they require more stabilization in the core and upper back, but the free floating open chain action is a heck of a lot easier on my joints.

That said, if your shoulders, wrists and elbows are healthy, and you want to lock in and crush some upper body hypertrophy goals (get after those show muscles), fixed bar dips are hard to beat.

Ring Pull Ups vs Bar Pull Ups: Which is Better?:    • Ring Pull Ups vs Bar Pull Ups: Which ...  
Rogue Matador dip bar review:    • Rogue Matador Dip Bar Attachment Review