Biggest dif is the y8 is OBD2 and z6 is OBD1. If you have to be street legal for SMOG testing (or what ever your state calls those tests I've never had to deal with before (ND and AK don't smog)) I don't think you can get away with using an OBD1 motor in an OBD2 car. The Z6 has the better block while the Y8 has a better head. Z6 block is a little stronger and came factory w/ tunable P28 ECU which is a huge bonus (though obviously you can use a jumper for OBD2/OBD1. The Y8 has the better tranny with closer gearing as well. All in all, stock for stock (no modification to either), they aren't that far different I don't think. Maybe someone else has a better answer "stock for stock". To get 250hp though out of either one, you better be boosting and now you're talking a whole new ball of wax. Tune, build, internals... etc
I have ZERO experience with the Y8 so i'm sure someone else will have a much better clue than I. I've run two cars with z6's this year and one will be boosted shortly but neither have been built.