They are definitely a different drive. I was having a crisis of choice and test drove a 2.2 Jag XF diesel and the Z4 20i on the same day. I almost went with the Jag because it FELT so much faster, it gave the jolt off the line and when speeding up from low-revs to overtake on the motorway - the Jag felt better every time.
On my second Z4 test drive, I said to the rep with me that it wasn't as sporty as I was expecting - he told me I was driving it wrong. I was upshifting around 4000 RPM - why? Because the Jag I'd just come out of had its power in the 1500 to 2500 RPM range, and my turbo petrol Audi has the turbo surge in the 2500-3500 RPM range, so I was used to changing up early to continue the power surge. The 20i BMW seems to have its power surge in the 5000-6000 RPM range. Let it rev through before each gear change and it will bring a smile to your face. So I agree with some above comments - you have to 'thrash' it compared to a diesel ride, but the engine is designed to do that, so its not a bad thing.
The upside of this is that motorway cruising keeps the RPM below the turbo window, which gives the very enviable 40-50mpg motorway mileage. Now whether this is what you really want from a sporty roadster is another matter altogether, if pure sport was your goal, the 35is would be the answer.
Also, if you got the 8spd auto, you'll never acheive a sporty get-away from the Normal mode, with 8 gears to choose from and economy on the brain, it changes often and early. If you need to beat someone off the lights, stick it in manual and paddle shift your way to victory.