For balance... if you have the opportunity to drive both - do so. The 128ti has quite different handling feel to the M135i - its definitely set up for fun. It has that puppy dog enthusiasm that my old mk5 GTI had.
Wrt to the gearbox - I much prefer it to the DSG box in the golf (albeit thats a 6spd box). The gear change points are much more intuitive. What I've found with the VAG gearboxes (both DSG and TQ's) is the hardware is fine / great but the software is not. BMW seems much better at integrating the two together.
Hard ride? It is firm, but much less so that previous Audi 'S' cars I've had. Also its not crashy and the rear suspension seems better set up that it is in the golf (7 to be fair). Somehow more sophisticated. Also, BMW have crazy high tyre pressures specified - around 3 bar. Take that down to 2.6 and its still got a sporty edge but totally comfortable day to day. Seats are subjective, mine has lumbar support and have no issues with comfort. Do agree with Fred that the M135 has better seats - would be great if they were an option in the Uk
Oh and the interior isn't made by fisher price and the tech works flawlessly
Very happy with mine. I do have to say the GTI community is stronger than the BMW one. Especially as true BMW fanboys shun the FWD newcomers...