Ring dips are a little riskier than bar dips due to the inherent instability. If you wiggle, you can put your shoulder in a bad position. Both have a reputation for bothering people's shoulders. You might be just fine with rings and I have done my share of bodyweight ring dips without incident, but I am not in a hurry to recommend either movement. I don't know how ring dips compare to bar dips on a rep for rep basis, only that ring dips require more effort due to the additional stabilization required. Weighted dips are probably best done from a stable platform instead of rings. I'd recommend close grip bench and pin presses before ever worrying about dips if you genuinely needed assistance work outside of straight benching and pressing.