I sort of made the reverse move, from RS5 to RS4, but kept the RS5 because I like it so much. I made the backwards move as it became clear that a 4-door car with a high revving V8 and 6MT is an increasingly rare commodity, and the RS5, of course, is shy 2 doors and the MT.
I think they are very different driving experiences but, if it tells you anything, I basically rotate back and forth between the two models on a daily basis (at least when its not snowing, as the RS5 is lowered and I won't put snows on it). I'm very happy with both. The RS5 feels quicker than a near-stock RS4, but probably is a dead heat 0-30, 0-60 and 0-90 with a tuned RS4 (at least at altitude, which I think you have some of). The RS5 motor clearly revs to the limit more quickly than the RS4's. The overall driving experience in the RS4 is a bit more visceral, and crude, than in the RS5, but thats not necessarily a bad thing. Even though I'm a 6MT person at heart, the DSG in the RS5 is a work of art that needs to be driven to be appreciated. The RS4 is pretty stealth in appearance, at least to those not in the know, whereas the RS5 (especially in Sepang blue) is more in your face and seems to attract a lot of attention, even though the model has been in the US market for 3+ years, which, if the attention is from women, and it often is, is not at all a bad thing. [I get paid by the comma!] My RS5 handles a bit better than the RS4 but that may be only because of some aftermarket suspension bits on it. Stock vs stock I don't think there is a whole lot of difference.
If you can, try to get a days worth of driving an RS5 and your RS4 in all road types and conditions. That might give you enough subjective information to make an informed decision. Good luck.
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