Hmmm, I dont think that first has owned both as he'd have been more than aware about the dashboards falling out on them if he had.
Both are gorgeous cars, and it epends what Z4 you're thinking of getting. The lower end models suffer from having something of a shortfall on power, once you approach the larger engine, the straight sixes are a far superior engine the turbo'd four in the audi. Manybe not in terms of outright power, but it has visceral charm that a turbo four will never have.
The Z4 is very much more the sports car and definitely has the better chassis, bringing the more expensive models to within a whisker of being as good as a boxster. Other details, like rear wheel drive, inline engine and the ability to drift the back end all day long make it much more the drivers car. Down to little details like when stationary and you blip the throttle, the car rocks from side to side like a big capacity sportscar should.
Thats not to say the TT is a bad car, far from it, but if you check out www.reliabilityindex.co.uk you will see that it suffers a host of electrical problems, odd considering every other audi has rock solid reliability. I mentioned the dasboards falling out, most have been corrected by now under warranty, but electrical gremlins are commonplace. The TT is the choice if you're not a confident driver, as a straight six powered Z4 will step its back end out at every oppurtunity, something you either love or loathe depending on your ability as a driver. The problem with the first generation TT is that is not a real drivers car. The new model has been hugely developed and is up there with the best, so if you're looking at a new shape then great. If youre looking at the old shape then be prepared for a golf in a party dress.
Ultimately, men would take the bigger engined Z4, women would take the TT if you want to pigeonhole it.
As an aside, have you looked at the Honda S2000? More reliable than either, build quality that no german company can rival and a howling 9000rpm engine. It swaps the Z4's charisma for clinical precision.
I'd plumb for the Z4 as long as it was powered by a straight six.
Course there's always the SLK........