Mate, I look at it like this:
I wouldn`t imagine the S4 is a car that you are going to track very much (apologies if I`m wrong, however) and if that`s the case, then why do you need the extra speed that a tuned S4 will give you over an RS4 ?
I`m not saying you`re one of them, but I`ve seen the video of Chris Harris`s cited a lot on various forums, and a lot of people seem to forget how easily the RS just romps away from the S before the S is tuned...
The RS is a VERY fast car, and is already much more than you can safely use on the road, and if you take into account the fantastic noise, the looks, and yes - sounds a bit naf - the extra "kudos" of owning an RS, then it`s a no-brainer for me.
Get the RS4 mate, you`ll have all the speed you ever need, and you`ll love it !