I'd have to say it depends. You probably will get more driving time out of the A6, as it's complete, running, and good to go. I consider my Z a lifetime project that I hope I never finish (meaning I will keep coming up with new mods not that it will remain undriveable ). If you are over dreaming up what your car could be, get the A6, it will prob save you alot of money/time/etc. If you want something that you MAKE into what you want it to be, keep the Z.
That being said, I had a similar dilemma a few years back before I got my Z. If all money was equal (and so far it's ending up that way) between my 260 and a new bimmer/370z/etc I know I would not wake up every day and look at a new car the same way and think "that was worth it".