Well, I'd have a 2.4 20v, tbh (funny that, seeing as I've got one!
Sounds great (for a derv - but actually rather good at full bore). 45 -50 mpg on runs down to 35-40 on shorter trips depending how you drive it. I never get less than 40 (real, not the over ambitious trip meter!)
I prefer the facelift look and interior too, though this is purely subjective and personal. Others prefer the earlier car's appearance.
The 2.4 20v has a WIDE power band and is EFFORTLESS to drive unless you're pushing on, where it can be a bit nose-heavy. Very quiet inside too. It loses out to the 1.9 in handling as the four pot is much lighter. I don't see many people getting much better economy out of the 1.9 150 though? I could be wrong!
The Lusso 20v cars (which mine is) on std suspension are awful on bumpy roads and bottom out for fun. It's possible to crack the sump. I changed to Koni STR-T springs and shocks and at 40mm lower it bottoms LESS! Still possible though. Oddly it doesn't LOOK particularly low and still clears square speed bumps....
The one real weak point on the 20v is the driveshaft inner CV joints. BUT mine is now on 90k and I only have problems with the LH one cos I missed a split boot and it got grit in it over the winter. Driveshaft UK can apparently sort them for £100-£150 per side with a better quality joint. Don't abuse the originals with hard take-offs and wheelspinning and they're fine.
The 2.4 is also really hard on suspension bushes / arms etc (or maybe it's my driving!!). If you're "handy" you can do it all yourself though.
I'm not knocking the 1.9 at all - for many people it's a more practical choice. After nearly 70,000 miles of ownership I wouldn't change my 2.4 for the world tho! Just need a downpipe decat and a remap to really make me happy.....
On either diesel watch for EGR valves sticking, MAP sensors clogging and VGT controller issues such as it needing a clean out or the small bore vacuum hose splitting. Upper or Lower boost pipes can split too and are best mended with Silicone ones.
Hope you find one, whatever you decide on. The 156 is a great car, just make sure it knows you care about it and it'll be fine