Well, nothing beats hitting your apex whilst turning into the gas station. It's a fun car.
The engine issue isn't about power - The 2.0T is a very able and, if I understand correctly, pushes the Avant to 60 .5sec faster than the E90 wagon - it's about character, especially in low-speed conditions. The BMW I6 develops power in a very smooth, effortless manner at low speed. The Audi's turbo 4pot on the other hand, exhibits harshness at idle and low speeds and feels burdened below 2k to develop power.
It's a genuine foible if you suffer through nasty traffic - Even though the 328i is slower, it doesn't ACT like it is.
Of course, with all the stop and go I'd probably be suffering even more at the pump with the BMW... Oh well. I'll be curious to see if BMW is willing to equip the F31 wagon with the 35i engine stateside. One can always wish!
As for MMI - There's just so much that it doesn't do. The B&O stereo is outstanding, but there's no graphic EQ, just 'bass' and 'treble' settings. When you're listening to a track or scanning the radio the MMI just sits there and doesn't go into any kind of 'standby mode' to display supplemental information (I realize there could be a setting for it - if so, I haven't found it). The corner buttons that surround the control knob are not intuitive in the least - the scheme reminds me of pre-smartphone UI - I wouldn't be surprised if the UX director cut their teeth at a cell handset OEM. Lastly, the buttons themselves have a very poor, somewhat delicate and high-resistance tactile quality - I don't know the technical term for the type of switch they've used, but it's not what you typically desire in high-frequency use buttons.
On the bright side - The Nav is outstanding. The 3d building mapping in the city is impressive and the binnacle center screen gives great directions. Also - the redundant MMI functions on this screen (playlist, phone, etc) work quite well.
EDIT Another thing that was a big deciding factor is the CPO situation. It was very difficult to find ANY CPO BMW sport wagons that had less than 20k miles and included a CPO warranty. You'd find lots of 'executive fleet' vehicles that had 7 - 12k miles, but no CPO, which usually puts you on the uncomfortably low side of being covered.
The A4, on the other hand, was the perfect scenario. 4k miles with less than a year since purchase with the addition of the CPO warranty giving me another 5+years/96k mi. of coverage. I sure wish it was an S4 wagon, but fee like I extracted as much value out of the transaction as possible...
On another note - The market for A4 Avants and cars in general is REALLY firm right now.