Coming from someone who's seen both....
They're two cars at almost opposite ends of the spectrum. Both are "sporty" cars, but one is SOOOO German and one is SOOOO Japanese.
Anyone who loves the styling of the Z, isn't going to really go for the BMW styling which hasn't changed much over the years.
As far as the interiors go, the BMW's do have a plain interior, but they are designed for the driver. Not the prettiest looking dashboard layouts but they are almost a study in pure function.
Throwing a few dollars into the 135 or 335 will beat a 370 in a straight line. Boosted engines are just easier to turn a knob and get 40 more HP out of it.
However, in the twisties the 370Z has a superior suspension layout and can be taken further than the bimmer. If you had a budget of $20K for either car, you'll be able take the Z further than the BMW's.
We know Z's!
and we know BMW's!
But our next car will be a................
370Z! 
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