bozy, i think it largely depends on what you're used to, what kind of mileage/driving you do, and what kind of cars you like.
if economy is your thing, the 1.9 diesel is king. if you want some massive ooomph, then the 2.4 diesel is best (200bhp, up to 210bhp from i think 2008 onwards). i've never driven a 159 with the 2.2 engine, but had the 2.0 forerunner to that engine in my old 156. if that was anything to go by, it will be very free-revving and sound reasonably good for a 4-pot, and have a surprising lick of pace for a normally aspirated petrol in a big car
the diesels are both ripe for remapping, and can become seriously quick machines. some on here have the 2.4's modified to well over 250bhp and 400 lb/ft. my 1.9 is running at 193bhp and 347 lb/ft - more than enough to seriously embarrass some supposedly much faster cars. remapping does nothing to harm economy either, in some cases it can IMPROVE it
downsides? the 1.9 diesel sounds just like any other 1.9 diesel. the 2.4 is fairly thirsty for an oil burner and can feel a bit nose heavy, and the 2.2 jts is also thirsty and (unless remapped) can suffer the hesitation issue so well documented on here. both diesels can suffer from a blocked dpf (diesel particulate filter) if used constantly on short journeys, as mentioned above
horses for courses and all that - go with whichever one suits your needs best