I would go m3 all day cause it is better in everyway. the only thing the s4 has is if someone want AWD. But then, there are ton of other better option for AWD and winter. X6M.
i even pick the x6 35i or 50i over the s4 in a heart beat. Since you are talking about winter, the x6 has more ground clearance, safer, bigger, more storeage, ton of power, etc.
or even the infiniti fx35 or fx50 over the s4.
IMO those SUV look more exciting than the S4 and more practical for winter.