A good place to look at reviews is
here. They will tell you everything you need to know...and some of it you dont. They even have example shots to look at so you can see the cameras actual output.
I like the Canon Ax series (I have an older A80 model). One of the big pluses for me is they use AA batteries. There are many times I've been out and had the batteries die. Just buy some AAs at a gas station or whatever and you can keep shooting. The A540 looks just like my camera with a higher MP count (Mine is 4MP). It doesnt have the swivel screen...thats just something you have to decide if you want it. Its nice to have but not really essential.
The camera isn't that big, but (at least mine) isnt small enough for me to carry with at all times. Plus I keep my tripod and shit with it, so maybe I'm not the best judge. That newer model is probably smaller since it only takes 2 AAs and my model takes 4. For a smaller cam I was looking at Canons SD models like the SD600 or SD700.
Really, you can't go wrong with either camera you've already posted and would be more or less my choices if I was looking for a newer point and shoot for myself. Personally I'd get the A540 because it can take AAs for those emergencies as a "main" camera. I'd *like* a SD600 to be my daily, carry it all the time pocket camera.
Oh yeah, heres a
review of the A540. Thats a good site as well.