just a guess here, but it seems the 27 and 25 in your model numbers dictates engine HP. so an additional 2 hp would be the start.
the Z and ZG are different in and of themselves. ZG is the next level above Z. probably heavier, stronger
S vs SKH is the next questions.
looks like a few. weight. about 500 lbs on the higher one. faster going forward and backward, the ZG is shaft driven instead of belt, has a wet disc clutch (stronger) over a dry disc clutch. mechanical lift vs the hydraulic lift on the ZG (deck) see through smaller fuel tank vs electric fuel gauge larger fuel tank on the ZG
there is many differences that I can see at just a glance. ask to ride both at your local dealer, the differences should be noticeable.