SD700IS vs SD800IS
Apart from the fact that I preferred the SD800 anyway, I decided on the SD800 based on the logic that I can always zoom in, but I can't go wider than my widest setting.
e.g the SD800 zoomed in one notch gives the same view as the SD700 at it's widest. It's only the very long end of the SD700 that the SD800 is missing (and you can always crop or use digital zoom if you really need to.) However if you're at the widest setting of the SD700 and you need to go wider, you just can't get the shot. (Ok panorama and stitching aside, but that won't work for 90% of shots).
Ok that's my twisted logic, but I very rarely need the very longest end of the zoom. So for me I like to have the wide angle option there (even if I rarely need it either).