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I'd say the Z still wins with it's lower price tag. Edmunds.com seems to agree-

When the 2008 BMW 135i came to us, we found it to be one of the purest expressions of the BMW character since the 1971 BMW 2002 tii. Now that the novelty has worn off a bit and we've driven one for close to 15,000 miles, we still think it's a brilliant car. With a long-legged engine that's as happy at 2,000 rpm as it is at 7,000 rpm, this BMW is destined to continue to win comparison tests and conquest buyers for years to come.

The 2009 Nissan 370Z achieves the same sort of thing on the Nissan side. It's so improved in every way over the 350Z that it comes off like a completely different car. Think of the 350Z as a really long prototyping program or maybe an elaborate focus group process. It might have taken six years to work out the kinks, but man, was it worth it. It is both sports car and sport coupe, just as in Mr. K's original concept for the 1970 Datsun 240Z and very much as the BMW M3 has evolved. And it wins this comparison test.

We'll even go so far as to say that the 2009 Nissan 370Z now raises the standard for the sport coupe segment to a new level of affordable excellence. Its accessible performance alone is worthy of status as a benchmark. Its looks alone will bring people to a Nissan showroom. And the combination of the Z-car's reasonable price and fuel -efficiency (22 mpg EPA combined) will certainly get your attention.

The 2009 Nissan 370Z has the whole benchmark package wrapped up with a bow: price, exterior design, interior packaging, performance, features and that X-factor that makes you just want to go out and drive one, and drive it hard. Thinking of the Z-car as a sports car is one thing, but we can honestly say that there's not another sport coupe that can touch the new Z. Dare to compare.

2009 Nissan 370Z vs. BMW 135i Comparison Test on Inside Line