The VZ has a lot of improvements over the VY especially the brakes. If you look at the 2 the master cylinders are different. The VZ's will pull up in a shorter distance and in an emergency even a few inches can make the difference between hitting something or mising it. The front sway bar links are different and the suspension is improved. The VZ SV6 will also have a 5 speed auto and if it's a later version active shift. The VZ motor is an overhead cam, 4 valve per cylinder engine and more modern. The 190kW also has variable valve timing on both inlet and exhaust valves.
The VZ also has an electric throttle (fly by wire) and the cruise control is integrated, you will notice the module that is in the VY's is mising as is the bracket to mount it.
The fly by wire system is better as it can vary a number of things one being the throttle position to respond to the accelerator pedal.
On the downside it would cost more to change things like O2 sensors because there are 4 of them.
With the VY is it a series 1 or 2? the Series 1 S pac only has a standard dash and 16" wheels. I do however think the front of a VY looks better, the only VZ front I really like is the Berlina and if I was going to get a VZ I'd look at an SVZ.