The Audi is a much more classy car than any Acura or Subby. The Subby WRX is a pretty good bang-for-buck car, but it is cheap b'cause everything about it "is cheap". interiors,trim etc.
But it is an excellent go-kart. Once the Mitsu Evo comes out early next year (in US), WRX will have a fight on it's hand in the same category.
Acura inspite of higher hp ratings (typical of Honda engines) is quite behind Audi or BMW in performance and handling and luxury. You gotta drive one to believe. Plus the Acura interior are kinda cheapish too and remind more of a Accord or Civic. But they are best in reliability.
Audi is releasing by this year-end the new A4, which has already been aclaimed to beat the BMW in dynamics in European test-drives and everything else was anyhow better than the Bimmers.
If you can afford the S4, there is quite no sedan under 50K, than can quite match up to it. Except when the new M3 comes out (but the M3 optioned out will be over 50K)
Meanwhile if your budget stretches to about 40K, you can even look at the Audi A6 2.7T manual. Superb car. Knocks everthing this side of BMW 540i/M5
HTH