These cars are all depreciating assets. The guys on the S5 forum are crazy if they are telling you that the RS4 is an out-of-date design. I don't consider the 03 RS6 out-of-date, and it's 6 model years old now. Also, when Audi adds their new suspension and torque control to the S5, does that make the current one old?
Which one you pick is up to you, but you are going to loose money either way if you are going to try and sell it. Mark my words, Audi will flood the market with A5's and S5's sometime in March of 09. They always do the exact oposite of what they should do. Bring a lot of the model out when it is realeased while it's hot and then taper off production. Audi does not do this. They make it impossible to get the model when it's hot and then when interest starts to subside they flood every dealer. Happened with the RS6, S4, RS4, and I guarantee it will happen with the A5/S5. Pick the car you want, not the one everyone is telling you to buy. My $.02