Right, I would say the 1.9 JTDm is the best choice, for an all round 'quick' 156, that is efficient and cheap to run and maintain.
1) It is much lighter at the front than the 2.4, it will handle noticeably better, and brake better.
2) The 6-speed gearbox in the 1.9 is the same as the one in the V6 petrols, just with different ratios, and is compatible with the Q2 limited slip differential, the 6-speed in the 2.4 is a GM gearbox and is not Q2-compatible.
3) The 1.9 is good for 190bhp-200bhp when remapped, the 2.4 20v is good for about 225bhp, but it has driveshaft issues and can't cope with the torque.
4) The 4 cylinder is much more efficient, although it doesn't sound as nice.
5) The sump on the 2.4 is dangerously low to the floor, you can't lower the car without really stiffening it up without risking the sump over every speedbump/hump-back bridge.
6) The 1.9 is cheaper to buy and much more prevelent.
7) The 1.9 is the same engine as used in Vauxhalls with 1.9 CDTi, there are a lot of tuning resources available for this engine (bigger turbo, custom exhaust manifolds etc) if you want to chase the big power figures. We have someone on here with a 1.9 JTD making a safe 250bhp..
So thats the reasons why I will be after a 1.9 JTDm as my next car. As much as the 2.4 sounds so much better, as a package it just doesn't compare to the 1.9, IMHO