Having just left behind an E90 M3, I'll have to say it was a level above the Z. What it lacked in head turning appeal, it made up with pure driving joy.
Whilst I wasn't keen on finally getting an auto, the DCT and high revving engine made it pure joy blasting on motorways and B roads. It was more fun being able to comfortably take 3 other adults with me on my journeys.
The V8, with an aftermarket intake and M performance exhaust was the best experience for my soul. If you can afford to buy and weekend one, do it. Well worth every sweet penny.
The Z was good, but it needs too much tinkering to sort the suspension and wheel hopping out. The BMW works fresh out the box.