Coming from a (fingers crossed) soon to be owner of a 350Z I still voted for the 2010 Mustang GT, because I think the 370Z is still ugly as hell.
Granted performance wise, If I had my choice I'd rather take the 370Z. The N/A V6 in the 370Z makes more HP stock vs the 2010 Mustang GT. Granted the Mustangs 4.6L V8 makes better torque of course, which would be more important considering acceleration. But... The 4.6L V8 gets garbage gas mileage so I'd take the 370Z simply because it offers similar performance with better mileage.
They both weigh around the same.
But for the price the Mustang GT can't be beat. (Except by a Camaro SS....
)
Both are great cars though. But yeah I won't be owning either any time in my life.