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.
2009 AW E92 M3 DCT
RAC RG63 w/ AD08R / OEM Competition Package Suspension Retrofit / Akra Slip On