Well....
The GPS has probably get the best forward pack out of any school representative team. Furthermore, the backline will be slightly better from the GPS than the CAS (would be far better if Jacques and Ontini Large weren't injured - I think Large can't play representative Union due to league however).
The ISA backline is probably the best backline out of any competition representative team - it looks lethal. But similarly, the GPS forwards are likely better than the ISA forward pack.
GPS should beat CAS convincingly like last year, however the game against ISA will likely be a lot closer than this years CAS game, although I think that the GPS forwards will get them over the line.
In saying this. Knox beat St Augustine's recently, which I believe St Augustine's were undefeated up until that point, even beating Joeys. But considering this as an indicator that the CAS is stronger than ISA or that St Augustine's is not the best team in NSW this year must be taken with a grain of salt, as it was practically a swimming pool at Knox 1 when they played, and wet weather rugby can often make the differences in the quality of teams far smaller than they would normally be, especially in dry conditions.