I can't speak directly to the 600, but I've used the SD400 quite a bit and find it a very competent little camera, giving consistently good results and nicely made. I wholeheartedly agree that a viewfinder, even a tiny one, is a big advantage in many situations. They, sadly, are being left off most subcompact cameras.
For $200 I think the 600 would be a fine camera. I'd probably poke around a bit to try finding something as good with a 28mm equivalent, but they're not common!