Hi
Just signed up this evening and, since I've had both, thought I'd chip in.
Had 57 plate Mk2 TT roadster 2.0 Tfsi FWD from about 18 months old for 3 years. Swapped to MX5 for 18 months and traded to a 2009 Z4 23i at the end of 2013.
For me, the Z4 is the better, more complete and nicer car. In truth, the open tops have only really been used as fun cars but the TT was only really of interest to me in that mode whereas the Z4 makes a very pleasant coupe too.
Positives for TT revolve around the cabin plastics which are nicer and less creak / rattle prone. Negatives were that torque steer was never entirely absent, undermining any sense of real security, the radio was rubbish, digitised and 'fuggie' yet the iPod interface sounded great. Ride was OK on 18" wheels but the car was no better in the snow than my 5 series, so winter tyres would be every bit as imperative for proper winter use. The roof seemed a little porous too despite Audi saying it was fine, so condensation was an ongoing winter feature. Gut feeling that service costs with the TT might be a little lower. Until, of course, the timing belt requires attention.
Xenon lights were an extra back then too, although our one had them and they were good.
The Z4 has 17" wheels and I've just swapped to non run-flats. The changes brought are considerable. Most of the rattles simply stopped and it rides much better. Ahead of the TT on 18's, I'd say. Fuel economy is better too, I think (early days) but probably ahead of the older TT. Later Audis will have doubtless improved. For me, seat comfort in the Z4 (sports seats) is a little ahead of the TT too even though I'm fat and the Z4 seats a little narrow.
The Z4 has the upgraded but not top spec stereo and it's powerful enough if just a bit boomy in the bass. It's mainly adjustable via tone controls but not entirely. I cracked and got the BMW integrated TomTom and it's better in most respects than the iDrive in the 5 series, if far less cool.