As another owner of both RS5 (and E93 M3) the RS5 is useless with summer tires in the snow and winter tires or all season are a must.
The RS5 and the new M3/M4 are 2 different cars the M will have higher torque at low RPMs while the RS5 is a high rev engine which is in my opinion is more fun. When it comes to handling I'm sure it will be neck to neck. The handling of the RS5 is breath taking if you will be brave enough to push the car
.
The RS5 has no adaptive suspension and the ride is rough and that will give the M an advantage when it comes to the comfort level.
The RS5 pulls much faster than my E93 (don't know about the E92) and since I got H&R sways installed I don't drive the M at all. At the beginning my plan was to get the RS5 until the ZCP package is offered for the M4 (2-3 years) but now I'm selling the M3 and keeping the RS5.
Don't make you mind until you drive both. With either cars you will not lose and for the winter just get a winter tires for the one you end up buying.
Best of luck!
