Which Canon ultracompact for a 2nd camera?

[katielou]

I own a Nikon D50 SLR, whch I love. But, I also want a smaller (MUCH smaller) camera. A fast response (very short shutter lag) is extraordinarily important to me, because I'm used to it and am too old to adjust. I bought my son a Canon SD 400, which I have used a few times and like. Now it's time to buy one for myself, and I am trying to decide between a Canon SD 600 and a Canon SD 550. Any advice? Or should I suck it up and spring for the SD 700??? Any other thoughts? I do not want to go larger than the SD 550. Thanks.