OP, the S5 is going through its mid-cycle refresh. MY13, due sometime in the fall, will have the 3.0T (supercharged V6) engine vs. 4.2 (N.A. V8). This will help performance while also improving fuel economy. It is a very good car but as others have suggested take one for a test drive. The current S4 will give a good feel for performance. It just lacks the design appeal of the coupe. Other improvements will include Google Maps and AudiConnect (i.e. hotspot). It is a very good car but, IMO, it lacks the emotional connection compared to an M3.
Assuming you don't live in the mountains or are challenged with driving in areas where you create your own trails in the winter, RWD with proper snow tires should be good enough. I drove several high HP RWD cars in snow and never had a problem. Does AWD provide an advantage, yes. However how many snow days are there per year, 4, 5 perhaps 8 (at the most).
If you like to modify your cars be aware that Audi is cracking down on this practice. Effective April they are running an ECU test. If modifications are detected it will produce a TD1 flag which will void the powertrain warranty. Also they are detecting intakes and exhaust systems. This will produce a T10 flag which can also void the warranty. Look at some of the popular Audi forums for this topic.
I had ordered a MY13 S4 but backed out to order a MY13 M3 coupe. For me, it was a better car. I just enjoyed driving it more. The S4/S5 are good but the M3 felt special.
The forthcoming RS5 looks really good except for limited allocation. Dealers will not be discounting from MSRP. Perhaps even a market adjusted adder.
The best way to determine is take both for a test drive on the same day (if possible).