I have had both cars, a '00 156 selespeed (2.0TS) that I sold years ago, and now the 159 1.9JTDm 16v.
They are close in terms of absolute power (hence,0-100 times are the same), but the sensation of power is more pronounced in the JTDm, due to the much higher torque. This makes it faster in typical roads, city, etc. The 2.0TS revs. very quickly, but you need to be above 3K rpm to feel something similar to an engine. Sound is beautiful and it is very refined, however.
Driveability is also very much improved in the 159.
So far, without the remap.
Now, with a remap (I have remapped mine one week ago, slight remap with +30hp, +50Nm), the 159 is way faster than the 2.0TS, with the plus of doubled economy, and it corrects the only defect (apart from the sound) of this engine: lack of low-end torque. Undertaking is a snap and you have power no matter what gear or revs (above 1500) you are in.
If you go for the 159 diesel, you will miss the TS sound, that's for granted. Elsewhere the 159 is much better, specially with remap and EGR deactivation. And I am a petrol-head, hating diesels until I drove this one!