I think they are two different types of bikes.. i don't know from experience, so maybe my vote won't count... however, i almost bought an 07 Z1000 before i got the sv650, but it sold a week before i could get set up financially.. the Z1000 is a sick looking bike, especially if you get rid of the ugly stock exhaust!
However, my next option was a new 06 SV1000S (which is still sitting on the showroom floor brand new now). But they were aksing too much for an older bike even though it was new.
So.. my opinion.. looks = Z1000, but the v-twin engine would be amazing in a 1000cc, for me, i don't think i could ever go back to an I4, so th sv1000 would do it for me.. but that's just me, both bikes are very nice! If you are looking at the S version of the sv1000, it will be better on the freeway than the Z1000 too