Z4 is 255 Hp
350 Z is 306 HP
looking for handling and zero to sixty acceleration

http://www.car-videos.com/performance/models.asp

I would have to say 350z by far. I am looking at the g35 and the z4 is slower than that so i would presume it is slower than a Z. Z is better in my personal opinion. Z is more of younger looking person car and is probably bought by younger people while the z4 is more elegant. Your decision. Hope that helps