Canon Eos 5d Specs
Besides price what are the main differences between the Canon EOS 5D and 450D?

I would buy the 5D, but then saw the 450D, which is very about specifications and much cheaper. Can anyone shed some light on what is the best model?

The 5D has a full frame sensor, This allows much more detailed and clear pictures. The 450D has a much smaller "harvest" of the sensor, the photos are not the same clarity and quality, even though the specifications appear similar. Also, the 5D is a generation that is much stronger than the 450D .. I would go for the 5D if I had to make a decision. Go to http://dpreview.com and compare the two side by side and read the reviews. There are test shots of both, you see the 5D superority.