Replace jockey on old XT RD-M750 or change out to RD-572?
I presume you are referring to the jockey wheels on the RD. These are not called 'sprockets'. They are not ceramic, though the guide wheel has ceramic bushings, while the tension wheel uses sealed bearings
The M750 RD is all aluminium and in comparison M592 is part steel (so slightly heavier), and uses cheaper jockey wheels. However it sounds like your jockey wheels are used up.
The other thing is M592 is Shadow which means no top pivot, which is a common wear point. Both are good, probably the shadow design is better, whereas M750 is a better implementation of the non-shadow design.
A new Deore RD will certainly be much smoother than a worn out XT RD. XT is more durable and lighter, but a worn out part is a worn out part.
If the RD has not had much wear in the pivots, but the jockey wheel has merely perished due to age then, you could just replace the jockey wheels in the old RD. But you don't say, so I'm assuming it's very well used.
The best possible solution would probably be a new 9-speed Deore XT Shadow RD, but I'd be happy with any Shimano 9-speed RD personally. (don't buy a fake one!) If you did get the M592, then you could upgrade the jockey wheels to XT grade ones for higher durability/slightly lower friction. The few extra grams from the steel cage is not really a big deal.