Downsides would be size. I've driven a tts and it's small as hell inside. The rear seats are useless.
If you dot need the rear seats, I'd rather have a tt rs than the 135.
In my experience Audi manual transmissions are better than BMW transmissions.
PCD 10/10 on 2015 M3 : MW / SB Full Leather / HK audio / Adaptive Suspension / Silver 19s / 6MT / CF Roof