I've driven both 125TSI and 103TDI cars. The transmission in both of them is very nice - the only issue I had with the TDI was (a) due to turbo lag and (b) due to me not being used to driving a turbo diesel in the first place.

That said, if the drive quality was the only thing deciding whether to buy a petrol or diesel, the TSI would win hands down. Its just that little bit more smooth and forgiving and the blast-off at the lights is a whole lot more