Which Canon?

[jiaozi]

Can anyone give me some input on which Canon is a better choice...mostly considering picture quality? The SD450, SD550, or the SD600? (I'm not including the SD630 since it's practically the same as the 600, except for the larger display.) I know the SD600 has the ability to shoot at ISO800 but it also has a smaller sensor size. I'm just not sure which is the most important factor to consider and which Camera is best.