By the same token, it's exactly the right time to be buying a S5 - you'll never get a better deal with dealer contribution, freebies etc thrown in as you will right now.
A5 platform remains the best looking (non R8) Audi, who cares if it's a few years old, still looks great IMO.
Every Audi has something in the class that's lighter and better, so I don't think that's a real life issue.
The cockpit of an A5, despite being older technology, is a much better place to be than a 8V A3. Some people place high importance on being able to sent texts via the car, having new sat Nav maps and connecting to the Internet etc, but that's overshadowed by the vastly superior seating position and to a lesser extent the integrated screen (I'll never like the pop up screen effort) for me.
B&O in the S3 is way better and I assume the RS3 will have the same system, so that's a definite plus point.
I'd suggest drive both. S5 with differential is a competent car to drive and will have better quality, albeit less modern feel to it. RS3 I expect to be a bit more brutal acceleration wise,
Driving the RS3 on Thursday, so will let you know how and if it compares to a S5.