I've used several point and shoot cameras over the years and one of the features that I find difficult to live without is the electronic viewfinder.
The Canon SX150 is a fine camera but I have been unable to get really nice photos when in bright sunshine because of the glare on the screen. It is also more difficult to hold the camera steady.
The Canon SX40 HS does have an electronic viewfinder and it makes all the difference in the world! You get an accurate view of what the lens "sees" and the photos turn out better.
Cameras without the electronic viewfinder are usually more compact and less expensive. For my money though, a camera with the viewfinder is worth the money.