IIRC the 159 is GM signum based, whereas the 166 is AR all the way
you dafty, the old 166 was AR, but so was the 156, again both were the same platform (tipo based). the 159 and facelift 166 are
both built on the "premium platform" that was jointly developed by Fiat & GM.
thats a good thing, it means it had more money and resources put into it than a purely fiat platform could. the 156 was good, the 159 was better, we should give thanks to GM because fiat couldnt have done it alone.
the 2003 facelift was the last chance the 166 had to make something of itself, but it still failed to impress.
i love AR, but the 166 was a mistake imo, it didnt do anything for AR's profitability or sales or image. you dont need to have 2 cars on the same platform, especially when you make one of them the ugly version. the only good thing the 166 did was make the 156 and 159 look better in comparison.
besides the 166 is slower (1sec 0-60 on avg, even the 3.0 166 was slower than the 2.5 156) and doesnt handle as well (more weight). and they wanted more money for it as well. it was doomed from the start, before you even consider the £50 per day depreciation. a new £30k car was worth only £13k a year later. ouch