In short, I preferred the Z4.
The E92, I bought this on a whim as I was looking for a new car and decided to just go for it (I made a post about it in May 2011). I owned it for around a year and sold it due to starting a long commute to work. I may have kept it a lot longer if I didn't have that commute.
You had to really wring it's neck to get it going. It did have some low down power and torque, but it wasn't effortless. That said, it was a very special car, the S65 was immense and it was so quick, most of time I felt it was too quick for the road. I had a custom exhaust (Chris Tullett) fitted by the previous owner. Made a big difference over the OEM exhaust so I'd recommend changing this as a first mod.
Mine had a manual gearbox, which was great but it could have been made better with a short shift kit. If I were buying again I'd go with the DCT gearbox. I also had EDC fitted and I'd definitely get this again. Comfort mode took out a lot of the bumps in the road and I left it in that for most of the time.
The biggest let down of the car for me was the interior; it was crap. It rattled and squeaked, bits fell off, the seatbelt pusher arms didn't work half the time and it was overall very dull. Fuel economy was of course, not good. I never got more than 22mpg average even when cruising at 65mph.
When I bought the Z4 I had some criteria; 2 doors, folding metal hard top, dual clutch gearbox, modern interior with a few gadgets, RWD and 300bhp minimum. I couldn't find anything else that came close to the 35i. There were hardly any for sale on BMW AUC so I had to go with what was there, but ended up with a well specced car (if I had the spec I wanted I would have had comfort access and EDC dampers).
The interior was fantastic, a much nicer place to be than the M3. Some of the plastic surrounds were quite cheap and rattly but not that noticable.
The N54 engine coupled with the DCT gearbox was a highlight for me. So much low down pull and the gearbox knew exactly when to shift. When set to Sport+ the shifts were lightning quick. It sounded pretty nice too, not too dissimilar from the S54.
I enjoyed having a convertible too, and because it was designed as a convertible it felt a lot more solid than an E93. The wind deflector helped a lot too, this is a must for doing over 70mph. I had a lot more memorable drives in my Z4 than the M3. The main reason I sold it was because I had gone back to doing long commuter miles (ironically I've now got a job just down the road
). The boot was plenty big enough too even with the roof down. My wife and I went for a weeks trip to France and back in it and had the roof down for most of the time. It got so much attention too, much more than my M3, which I was really surprised about. I covered 20 odd thousand miles and only went through 1 set of tyres and one service. I had one coil pack replaced under warranty and that was it. I also averaged ~28 mpg over ownership, and I don't drive like a granny!
If 4 seats aren't a requirement and you fancy some open top motoring then I couldn't recommend the Z4 enough. I'd push for a 35is as they have a bit more power (same as the 1M Coupe) and the adjustable dampers are standard. I believe they have a louder sports exhaust too.