for buick, a vr engine is the best option, stock has higher comp ratiio and a few other internal improvements over the vn-vp buick
if going for FI then a buick has stronger bottom end and a vn-vp with lower compression would be the pick
ecotec if your staying na would be a good basis but is a bit of stuffing around changing exhaust and mating the earlier auto trans as the bottom brace from the bell housing to block differs....also these are generally lower km engines
Either v6 by nature produce their power down low, although the ecotec revs a little bit harder it's still not a good idea to rev either engine high, the split big end journals are a weak point on the crank and there's no way to change it.... higher than 6000-6500 rpm on a stock engine to often will end with something in the bottom end breaking, if money isn't an option you could import an aftermarket alloy block with 4" bore, stroker kit and turbo/super charge it to build a 1000 hp monster but it'd work out cheaper to simply drop and LSx engine in there....
At the end of the day it all comes down to what you want to do with it, both cost about the same to rebuild and they cost more than a 304 to reco......