Well, you all know might thoughts on Aweomes Stage3+ A4, BUT...
I've never driven a B6 S4, let alone a new RS4, but I would hope that they were both faster or at least felt faster on the road than the APR A4 because of the lag (not helped by the lightened flywheel, I think). In the APR A4, 6th is useless below about 100. Owners might occasionally track an Stage 3 A4, but for the money it's not the first car you'd pick as a track day car.
I think the issue is that you can spend £20k and have a 1.8T Quattro (I wouldn't do it with a FWD, although I'd be interesting to see what one was like) that's got 360 odd bhp and goes like stink, something a bit special/different and perhaps a car that you can use as a bit of a project. Or you could get a V8 S4, with dealer warranty and have the S4 interior, etc etc
Or if you want an RS4 for 20k, get a B5.
Again, I'll state clearly, I adore the Stage3+ A4. If was going to BT a car, it would be an A4Q as I have a philosphy about not having "weekend specials" when it comes to cars/bikes. Mine need to be driven every day, or there's no point. But, when you look at the cost (and it's not "expensive" just "high cost"), you're back to looking at S4's. If I was looking for a track car, it wouldn't be an A4Q (I don't think it would be anything VAG).
So, it struggles to fit in any category, if you catch my drift. An A4Q with a K04-23 might be a better option for a road car.
How far off topic did I just go
?