Be sure to take the Z on some curvy roads. The steering is to die for. I've never felt anything like it.
If you've been looking forward to the Z test drive, it sounds like your decision has already been made.
I have to pipe in on driving the Z in stop and go traffic. I've only owned manuals, and the Z is, for a manual, extremely comfortable to drive in stop and go traffic. The gears are very forgiving. You can do a slow roll in third. The clutch is fabulous. Every time my husband drives my Z he is sick with envy (he has a Mazdaspeed 3, the most unforgiving and temperamental clutch known to man).
The V6 is so utterly smooth no matter how you're driving.
But I have to say one more thing about the GTI. It will be a lot safer in an accident. I got clobbered in my Golf (predecessor to my GTI) and walked away. The car was crushed to hell, but I was fine. VWs absorb a crash very well for a small car. I can't say I'd be as safe in my Z if I got hit as bad... And another thing you mentioned: visibility. Visibility in a GTI is great. Visibility in a Z? On my first test drive I felt I was driving blind. But that will go away. You'll adapt. I slid through traffic now in my Z and I feel I don't even need to look (although I do anyway). That goes back to the whole "extension of your body" thing.
I still say Z! Z! Z! Z! hahaa...