First off, I have never driven a car more fun to drive than my Boxster. I'll admit the pride of owning a Porsche is an added bonus. I've driven the 350Z and it definitely has more low end torque than at least my Boxster, I wasn't able to hit the twisties so I can't comment on handling.
As far as a BMW, I have a 330ci with the sports package. It's a totally different ride. Even though it has the sport package it still feels like a luxury car especially with the passenger seats behind me. I recently rode the BMW from NorCal to NOLA and back. I'd never try that on my Boxster.
So what is comparable? If you take the Porsche experience into account I don't think any other car can compare. For me, riding the Porsche top down on twisties is like riding my motorcycle carving up canyons. Waitaminute... that's my answer... a GL1800
