Hard to answer in a straight forwards way. Stock without a LSD the 335 really struggles for traction, in 2nd gear unless the road is slickly smooth the traction control would kick in. On your average poorly surfaced B road its almost impossible to really enjoy the car because of the lack of traction. Hence I got a LSD fitted to mine almost as soon as I bought it, and now the car behaves the way it's suppose to.
In terms of pure grip I think the 2 are very similar. The 335s damping isn't as good as the Z's, but that might be down to the runflat tyres. Overall I'm very impressed by the 335, and cannot really fault it's handling.
But in terms of driving "fun", I find my self actually taking the 335i for longer drives than I did with the Z!! Only difference been the 335 can carry 4 adults and all their luggage Take one for a test drive you wouldn't be disappointed.
I loved my Z but have to say I love the 335i even more, despite there been a 320D around every street corner Theres also a ridiculous amount of tunning options for the 335 most of which is not that expensive, mainly thanks to the Americans who seem to love trying to push the N54 engine as much as possible (there's a video of one on YouTube out dragging a Lambo ). Have to say its going to be very hard for me not to buy another BMW as my next car!!
Interesting. Coupe or convertible? Or the hard top which folds into the boot (what's the name for that)?