my opinion... i own a vz sv6 with a charger, makes 315rwhp and 872rwnm in 3rd, my mate has a vs (ecotec) with a charger, unsure on power figure. i have driven both and can say this... my vz as being better than other vz does not pull anywhere near as hard as my mates ecotec.
the drive by wire is laggy, as in a few milliseconds delay when driving, very annoying as i notice this, some people dont notice, but its there.
the alloytec is a lot more expensive to modify, and not much in the sense of aftermarket support.
the alloytec is a lot more expensive to repair, but in my opinion, my car looks nicer.
the ecotec is a reliable engine, as with the alloytec, but they both have their own problems.
ecotecs leak like a siv, the alloytecs do not really have that problem, except in later life like high 100,000km.
the alloytec is a very strong bottom end engine, as over in the states, alloytecs have been able to make 1300hp with standard bottom end.
ecotecs sound nicer in exhaust wise, but can sound bad with the wrong exhaust.
after all this, if i were to do my time again, i would buy an ecotec, simply put that they are a very reliable engine, the transmission is better than the 5speed auto on the sv6, the aftermarket support is a lot better for the ecotec and is cheaper for them too.