I made this transition 2 years ago and to keep this short and sweet, there are two things I noticed
1. Hot summer days the M3 is a far better drivers car and is the quickest across country without any doubt, wet days and winter days the S4 is quicker and a less scary.
2. Running costs for the S4 are drastically reduced compared to the M3, every time I had some work done to the M it would be a bill for £1000 min, S4 is almost like running a Mondeo with regards to costs.
I worked for Merc for 3 years and had a C63 AMG for 9 months, now that was different again, I went through 4 rears tyres and 2 fronts in that 9 mths, servicing and running costs were irrelevant as I had it for 9mths.