Well if your going for a bone stock longblock, the z6 has slightly lower compression. and would possibly be easier to tune. However the y8 has a better flowing intake manifold. If your building them up then the bottom end isnt a big issue, new pistons, rods, and some sort of block guard/fill/sleeve will probably negate any difference they have. The thing youd be more interested in are the heads. The vtec heads will flow better and provide better top end power stock for stock.