I'm just going through a similar choice. I'm after a new A6, I've got to make the choice between the 2.0TDi and the 2.7TDi. I'm erring on the side of the 2.7 right now, because although I will get a few more MPG out of the 2.0, if I drive it to try and get it to perform like the 2.7 I'll be getting better MPG out of the 2.7.
I can vouch for the 150bhp TD engine, my A3 currently uses one, and the performance is excellent. Don't expect to outdrag a Civic Type R off the lights, but for overtakes and motorway driving it's hard to beat.