This was my dilema also - TT-Tdi ot 123d coupe. I went for the BMW - reasons:
- Engine - the BMW is smoother and has more constant power (and more of it) right through the rev range. The TT is improved over A3's ive driven but still not as good with a 'band' of power outside of which you are left trailing - fine once you are used to the gearing... but not on a wet day!
- cheaper to run and insure
- No torque steer - especially important with the diesel! Even with the 4wd i'd expect most of the power is at the front (I ordered months back - was only able to drive petrol)
- I've had both makes and found BMW customer service to be far better - though I do agree that the build on the Audi is of a higher standard.
- 123d is a lot less likley to attract negative attention
- 123d makes more sense on a practicality basis (just!) and was the more comfortable of the two in ride and driving postion (i am 6ft3)
The audi does have the better interior, and probably is the better looking (though the original is better)
There is no bad choice here - more style or perfomance than anything. You could always wait for VW to launch the same setup in the Scirocco and save yourself £5k!