The B8 S4 is a great car. Blown 6 cyl is the way to go.
How much was the RS4? I've seen them in the $40k-50k range and that is way too much for a car with an exterior/interior design that dates back to 2002 - it looks really dated and the interior looks like an A4 from the 90's.
For my money I'd get the e90/92 M3, equally potent, high-revving V8 and similar in price.