450 is pre '08 [in the US - cars before there were US smart dealers. in the US these will be GRAY MARKET cars oooooh.....buggy....buggy..buggy]
451 is '08 [in the US - cars from 'official' US smart dealers]
The 450. I've been driving a GRAY MARKET 2005 passion fortwo coupe 450 in L.A. for year and half . been having great fun. it's cuter, more loveable, has a daimler-benz engine turbo w/ six speed trans instead of that mitsubishi junk, and it gets better gas milage - avg. 42mpg. It's the car Americans thought they were going to get.
insurance: no problems
title and registration: no problems
loan: no problems
parts and accessories: no problems
service: no problems
no "Dealers": OMG!, there are smart dealers in the US now? Who cares? Who wants to be held hostage and gouged by a dealer?
The 451. kidding aside, for an impartial list of 451 improvements see:
Evilution - Smart Car Encyclopaedia
If you are going to be a smart owner you must know about evilution.