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The toy makes all the difference.
Picture two sets of dips. One on rings, the other on a quarter ring: basically a bent plastic pipe with a nylon cord through it.
The difference is subtle. A full ring is more rigid, and you can pin your forearm against the top of it while you grip the bottom. That brace makes the setup relatively stable. The quarter ring is lighter and rotates freely, with nothing to brace against, so it leaves all the stabilising to you.
That instability is the whole point and a double-edged sword. The free-rotating handle does two things at once: it lets you sink into greater depth and a more lengthened position on the pec, and it strips away the stability. So you're more likely to fail because you can't hold the position than because your pecs ran out of gas. The shoulder gives out before the chest does.
Which means the instability quietly relocates your failure point off the muscle you're trying to work. The upside: as you learn to stabilise, the constraint shifts back toward the pec and you start working the chest harder than the stable setup ever let you. Ideal world: you don't choose. You can handle more external load with a fixed bar. And you can train deeper positions with a tool like rings or handles.