The most obvious difference is the engine, 3.3 vs 3.0, I've not driven the 300, but I would wager that the 330 drives nicer. From what I've read here, they both have the acceleration hesitation problem/annoyance, but my impression is that it is more prevalent in the 300. Lastly, I recall people complaining about the 300's wide turning radius, that of the 330 is much improved.
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I would add that the body styles are little different Alot more curve from the front to back. I if im not mistaken the 330 looks a bit wider and longer too. The rear tail lights are higher up. The 330 is 5 speed auto 300 is a 4 speed. In my opinion I think you be happier in a 330. I currently own a 300 luv it but when the time is right im going to trade up to a 330 or 400