Redbook layer on SACD - good enough?

Buy it. No difference unless the manufacturer intentionally wanted to make one.

No. How could it? But it's likely that the mastering is different on the different disks--and there's no guarantee that the best version (or, rather, the one you would like best) is on one or the other. If both the CD and athe redbook layer of the SACD used the same master, they will likely sound identical.

DSD layer from SACD Hybrid wins over Redbook CDs. And more important is SACD being played on SACDP sounds lot better than DSD layer to be played on Redbook CDP at the same price range, or even more expensive.

This is a fact.

"And more important is SACD being played on SACDP sounds lot better than DSD layer to be played on Redbook CDP at the same price range, or even more expensive."

How do you play a DSD layer on a Redbook CDP?