I understand that rings and a perfect pull up (the ones with rotating handles) are very different. Although, the basic difference I see with it is that rings are FAR more unstable.
If any of you cheapies out there happen to have one of these, then you can use it (until you get really advanced) for ring strength!
Just a suggestion. In case you don't have rings yet
I'd imagine that they would be the best possible substitute for rings.