Handling wise the ZS will be very close to the ST cause they are not that different in suspension setup. Saying that lowering a zs will make it on par with the st is ok but what happens when the st's been lowered too, which 90% of the ones I've seen have been...

Had a zs on the motor way a few weeks back who was travelling at the same speed as me, I didn't notice he had dropped it a gear and he started pulling away, so I dropped it a gear and started to wave goodbye to him out the window as he dissapeared into the mirrors lol.

The only reasons I can think of going for a zs is obviously if your on a tight budget or insurance is an issue or you want a Diesel etc...