In other words, not much difference at all. However, I disagree with your opinion about aiming with the 12.75 vs 11.75. Although it may true for you, it depends a lot on how and where you hit the cueball.
Yes. Not much difference imo of "actual play" between Z1 and Z2. I have both of them in front of me right now, and can switch seamlessly between them.
As far as physical differences, the Z2 has a shorter ferrule of a harder material, and a phenolic plug where the joint insert screws into. This is all common knowledge if you were a predator fan when they switched from the 1st gen to the 2nd gen shafts. It was in all of predators marketing info, and I'd imagine it's still on their site somewhere.
Predator did claim the 2s had slightly less cb deflection than the 1s.
One benefit of the 2nd gen is the ferrules are less likely to become cone shaped from when the ferrule follows through and rubs the cloth, due to the material change. I did cue repair for a few years and this was fairly common on the 1s, especially if the player broke with it.
Good luck in your search. I'd personally get a 2, mostly because it's newer, and you are probably more likely to find a used one in excellent condition, if you are looking for a used one.