Gotta say, the reliability stories on the B7 are scaring me more than anything I've read on E9x....
For personal experience, I think B7 has nicer interior than B8, and nicer in terms of quality than e90. It also has much better seating position for my liking. It understeers as a pig vs BMW which is natural given they are driven on diff axes and fwd nature is annoying after 7 years. However I think quattro isn't so bad and sure footedness of it it winter is unmatched by rwd E9x.
It's a hard choice - for now it's hard to go towards Audi after I've seen the prices quoted on them for repairs (i.e. if all main components that tend to fail - do fail - it is like price of a car to put them right - that's crazy).
'07 TiAg E86 Z4 Coupe 3.0si