I have found my way here during my search for a new car.
I currently have a Mk4 golf gt tdi 130 (mapped to about 170).
I have just driven a 120d m sport. Nice car. Liked the chassis, and even like the way the 3 door hatch looks (not to everyone's taste I know).
I reckon I'll be buying one soonish, but have been reading about the 123d aswell.
Will post up in the relevant sections for advice etc.