judge them as you want:
1) y7/y8 oil pumps are crap
2) y7/y8 heads have quench pads
3) z6 has the best flowing stock oil pump of d-series motors
4) y7 has vtec sized valves
5) y7/y8 have small ports
6) vtec is useless in a boosted d-series
7) z6 crank has four oiling holes in the crank vs the y7/y8's two holes
8 ) y7/y8 have bearing issues (spinning them)
9) y7/y8 mini will net more CR then a z6
10) y7 has the same design IM as a DPFI d15
11) s20 a/b000 trans has aluminum shifter forks (spares the gear set when they break)
12) s40 a/b000 trans has steel shifter forks (ruins the gear set when they bend)

personally, built for boost z6 block w/ y7 head and a bisi stg.2 cam & bisi springs. and then of course add boost