BMW 3 Series: xi VS. i?

I am looking at buying a used 3 series '06/'07/'08. I'm looking for a performance essment of the difference between the AWD 3 series and the rear wheel drive... (I live in an area with a couple annual snowstorms and an inch here and there in between, and I live at the top of a decently steep hill so I want to know how much of a difference there is between the two).