I think the poster is using the 'imperial gallon' as many in the UK and Canada still do.
Frankly, the 450 seems to be much better built than the 451; probably because they built it for almost ten years. Ours, bought new in Guadalajara for $13,700 USD, and one of the last 2007 models, has been absolutely trouble-free. We average the equivalent of 40-44 MPG and have done nothing to it beyond the normal 15,000 Km service and a subsequent 'annual' oil change. It is disturbing to read of the many problems encountered with the 'USA version'. Surprisingly, we don't see these same problems with the 451 European version sold in Mexico. It would be interesting to hear from our Canadian friends, as I suspect that they are now getting the European version as well. The only advantage that I can see with a 'grey market 450' would be the desire for an 'original smart'. However, I would want to live close enough to Canada or Mexico, preferably Mexico, for service and parts that might be needed from dealers who don't discriminate and who know both versions well.
Let's put it this way: If we moved back to the USA, we would have to sell our 450 in Mexico but would probably not buy a 451 in the USA until all the 'bugs' are gone.