On the strength of the top gear comparison I test drove both. The 370z is a bargain, has lots of kit (black edition) and is very reasonably priced. It is however quite a bit more expensive to insure, and at 12-15mpg around town also a lot more costly to fuel, at least for me, as I was looking for a daily driver and a fun car in one. 370z also has lots of torque, but the engine does sound a bit dull, there's no exhaust note to speak of for such a large engine, nothing throaty. Handling was also a bit twitchy on city roads and the suspension is very hard. Still, it was a very nice car, looked stunning as the black edition and if I were 25>30 I'd have that in a heartbeat. But I'm a bit older than that, still want the power and fun, but also want some comfort and style, hence the Z4 35i.
I'd thoroughly recommend the 370z to anyone wanting to be thrown about by a bit of muscle (no Mark/Testes comparisons please lol).