This is tough. It's like choosing which is better, man or woman (not in a pervy sense). The 156 is quite feminine to the 159's more masculine appearance.
It's also easy to think of all the shonky, badly maintained 156's on the roads and in the scrappers while the 159 is still pretty fresh. I had a 159 and loved it to bits so I'm feeling like a traitor going 156 now. Remembering what they look like when immaculate. More important, to me anyway, it must have been one of the most eagerly anticipated launches from when I first caught sight of the design leaks. It was so achingly gorgeous for a road car. It's the winner.