I know it is faster, but I couldn't order the 123d at the time I ordered mine. Or, it should be faster, but the difference is smaller than you'd expect. The top speed is a problem with the 123d, they seem to be limited to some 234 (real) km/h (almost all 123d have the same problem), so that's only some km/h faster than a 120d. And since the 123d's fantastic engine is (for a diesel) so linear, it doesn't feel faster in acceleration. It's got a fantastic linear feel (i've driven it on several occasions), and as you know, the less linear the power curve, the more powerful it feels (nice example: Volkswagen Golf IV 1.9 TDI 150hp).
I'm glad with mine, and from the moment the BMW/Bosch EDC17 checksum- and writingproblem is solved (chiptuners are having a ball on this one, it's very heavily protected), I'm going to have it chipped. 215hp and 450Nm