I looked at an Impala on the lot recently. The thing is huge and to my eyes, rather bulky looking. The bulging hips do nothing but make it look bigger. The interior is typical GM overstyle. It just looks ostentatious. The front end of the Avalon is odd looking, but I don't mind the profile, which is similar to the Impala only sleaker. The rear end of the Avalon looks better to me also Two litres per 100 Km difference in fuel economy is nothing to disregard and the drive train of the Avalon is excellent. So, the Avalon wins using my preferences.
That being said, I may be in their target demographic, but both of these vehicles are old mens' cars to my way of thinking and I wouldn't drive either. Maybe when I'm 10 or 15 years older, I'll soften a bit to these land barges. Right now I need a compact wagon or even a CUV, if I had no choice, for my active life style, and preferably European engineered to better carve out the corners without much body lean. Happy motoring!