I like Audis. I like their style, their performance capabilities, their "personality"... much nicer interiors than most BMWs.
But I don't think I'd buy a current Audi. Well, I don't think I'd buy many of the current Audis. They have worse carbon buildup problems than BMWs tend to have. They also don't feel as nice to drive as BMWs do.
Remember, BMW's mission lately hasn't been "the ultimate driving machine" but the "ultimate all-around machine" and in this case I think it succeeds. Take the M3. You can take it to the race track just as comfortably as to the grocery store. Same as the 1 and 3 series in their more performance oriented variants. You can beat the hell out of the car, and it still wants more. Sure, a Porsche can easily beat out a 1M or M3 on the race track (not even mentioning a regular 1 or 3 series), but there's no back seat and you feel like you're ALWAYS on the race track, even when you're on the street. A BMW can take daily abuse, provide plenty of performance capabilities, have enough cargo room, allow for people to use the back seat, be commuted with every day, be taken to a track on the weekends, and much more.. and it performs all these tasks admirably. Does the 302 Boss Mustang equal the M3 on the track for less money... yes. But are you equally comfortable commuting with it or just cruising around day to day? No way! I think that's what BMW's mission has been lately.
Audi on the other hand... The 3 series outclasses the A4 in most ways. The 5 series outclasses the A6 in most ways. The 7 series I don't like much, but in that price range, Mercedes' S-Class spanks the offering from both BMW and Audi. Z4 vs TT vs SL(K) I don't really have much experience with, but BMW's current lineup of SUV's is at least on par with Audi's. As for the sporting variants, it's very hard to say because the Audi lineup is structured VERY differently than BMW's. A 335 will spank an A4, which (in stock form) will be spanked by an S4, which will then be spanked by an M3, which... actually it tends to beat the RS5 in comparison tests.. (http://www.motortrend.com/roadtests/...n/viewall.html
It's very hard to say. I always thought of BMW's as more sporting focused while MB is more luxury focused and Audi is smack in the middle. However, BMW has been moving towards pleasing more people lately as they take up a greater market share than ever before. They are becoming the "all around machine" which isn't terrible.. as long as they keep them just as fun to drive. Audis also tend to be a little more expensive. My dad is shopping for a hard top convertible. The A5 is a soft top, and Audi doesn't really offer anything in his price range. He wants a 328i vert, because the VW Eos and Volvo C70 aren't really refined enough. Mercedes' SLK line is too expensive and only seats 2 and their other verts are way up in price. BMW is kind of becoming a "people's sports car" just like the original Mustang was, except I guess one price notch up. Can't afford a Porsche (or can, but you need a car that can actually function as a car with a trunk and back seat?), or want a sporty car but can't afford a second one other than your daily driver?... get a BMW. That's what the 3 series in particular excels at.
Also, Audi is huge. BMW isn't. Audi is owned by Volkswagen and shares many parts with them, but is also owned by the same company as Lamborghini and Porsche and benefits from that. BMW has Mini and Rolls Royce. There is no part sharing, but there also isn't nearly as big of a backing to them.