350z HR - again, biased, but had same decision as you.
Just can't get away from it - hate the way the 370z looks. Compared to the 350 is has lost some of its natural good looks IMHO.
And, engine wise, the HR engined 350z is essentially barely down on power compared to the 370z. Test drove both, couldn't tell the difference. In fact, the only discernible difference for me was that the 350z was a) better looking b)slightly cheaper and c) a bit more engaging to drive and 'connected' to the road.