I own multiples of both. They're obviously both great scopes but there is no comparison optically. The Z3 is much easier to get behind and provides a clearer, brighter image to my eye. That said I don't think you'll miss an opportunity at an animal if you use a VX3i. I often shoot each scope back to back at the range while the barrels cool on different rifles and the optical difference becomes apparent quickly.
I've never had problems losing zero with either. Both track OK.
Only downsides I see with Z3 is eyepiece is larger and can leave less room to operate a bolt with gloves on. I use the Z3's on 3 lug guns with low bolt lift for that reason.
