I haven't owned either a VY or VZ, but i've done a decent amount of driving in a VY, and also driven a VZ. I was quite dissapointed with the VZ alloytec. I was expecting it to be much better. In my opinion the alloytec in the VZ is a "downgrade" on the old ecotec VY. That might sound strange but let me explain.
The VY was much more responsive off the line, where as the VZ you need to rev the nuts off it to make it move. That wouldn't be such a downer if it sounded nice, but the alloytec sounds more like my mum's 30 year old sewing machine than a sweet revving V6. (eg: any toyota V6 of the last 20 years or even late 90's magna V6's :rofl: are far superior to the alloytec IMO!).
The VZ alloytec has a fairly even torque spread. It makes close to its peak torque in a lot of the rev range which should be a good thing. But the feeling I got after stepping out of a VZ, this translates to - gutless everywhere.
I wouldn't say the old VY ecotec is a "superior motor" to the alloytec, because its really not. Its a bit of an old dunger really, but it does the job quite well and feels less gutless and more responsive than the alloytec. The downside of the ecotec is that its a pretty harsh motor at high revs. eg: it sounds like a conrod and pistion could come through the sump any second above of 5,000rpm, but if you don't thrash it then its not really a problem. But even still, the VZ alloytec isn't a smooth revving motor either. It is smoother than the ectotec, but still a harsh revving motor by todays standards.
So in conclusion, the VY gets my vote. It sounds better, is more responsive at low revs, uses about the same amount of fuel as the alloytec anyway, and isn't any slower. Just better to drive really lol. I may be slightly biased because I had a VS ecotec for a couple of years which was a good car, but I'm sure there'll be some VZ owners who are biased towards VZ's too, and there's nothing wrong with that either. Drive both and see which you prefer.
Wow that was a ramble and a half! I do that when i'm tired haha. Hopefully that gives you something to think about anyway.