powershot sensor size etc

I am looking for a high quality easy to carry around camera and at the moment its between the powershots S3,A710 or A640.

Whats the general thoughts on these models remembering I have good experience with DSLR s.

There is one thing that baffles me ...the sensor size either 1/1.8 or 1/2.5
which one is the biggest and what would you go for ..and why ?

Thanks Guys

Sensor sizes. 1/1.8 is the bigger of those 2. My "carry around" camera's the A620. Unless you're pixel-hungry, maybe go for the A630 instead of the A640 - same size sensor but fewer pixels, so possibly less noisy.

Sensor vs. other similar cameras was why I chose my A630 ... however, low light i find to be the pixelly area (badly) - noise ninja to the rescue

Thanks for the replies seems I save some cash that way too.
Is the noise a real problem in comparison to the competition.Everything else looks very good.