Hi,
You may want to fully read the reviews on both, which is too much reading for me at this moment:
http://www.steves-digicams.com/2006_reviews/sd630.html
http://www.steves-digicams.com/2006_reviews/sd600.html
I notice that Steve's Digicams places SD630 on their "best cameras" list in 6-8 megapixel category, over SD600.
Another site has SD630 (again, not SD600) in their "top click-through" list, suggesting that there is a greater interest in SD630. Neither of these alone would make me choose one over another, but it's worth researching why.
The consideration about LCD screens being practically useless in bright sunlight is valid and there have been times I've found the optical viewfinder on my camera handy. However, keep in mind the viewfinders on small cameras like these are not terribly accurate as far as correctly showing the boundaries of the picture; they usually exaggerate zoom (the picture will have many more things in it than what you see in viewfinder) and are slightly off center.
