mate, seriously. I've owned a 1.9 JTD M-Jet 150bhp Sport 156 and I now own a 159 1.9 JTDM 150bhp. Stick with your 156 at all costs.
The 159 does at least 10mpg less than my 156 did, is bigger, heavier, more costly on tyres and all the rest. I do more town work now than motorway. Last motorway run I did the 159 returned 44mpg. The 156 would return 56/57mpg. I could ge the 156 to maintain 45+ mpg around town. the 159 is giving around 35mpg aronud town.
A good set of second hand lights will go on easily and they're not that hard to change. you will have to replace the clutch on any car at some point - you'll find yourself changing it sooner on the 159 as it is heavier. You change the clutch on the 156 and you know you're good for another 100k miles.
The fact that it needs new discs and pads shouldn't be a reason to sell it either! haha. You can probably change them for around £40.
The 156 is a more drivers car, it is more nimble and fun to drive, where the 159 doesn't give the same positive lively feel, it is more GT cruiser, but the 159 will have more ultimate grip if you've got the faith to shove it in.
My personal preference though, even owning a lovely white 159 right now, would be if I could I would go back to a 156, they're cheaper, parts are cheaper, running costs are less and the driving experience to my mind was much more fun.