The AX brakes predates Campagnolo's Delta.
I have a couple more differences 600AX versus Dura-Ace AX:
- on the backside the 600AX has two generic nuts, while DA AX has specific manufactured examples
- personally I like the way how Shimano sculptured the DA AX better
- (I think) the DA AX came from the factory with a deeper gloss
- the DA AX callipers have a longer cam
I have pair 600AX brakes in use on a rider. Personally I think they perform satisfactory, but availability of hoods and pads is an issue.