All of these are Canon cameras. Each has its own benefits. The 300 HS is very small and will fit into a pocket or purse very easily. The S100 is a bit larger and heavier, but has the ability to take better pictures in low light. It also has RAW format, which means nothing if you are looking for a camera to set in AUTO and snap away. The G1X is nearly the size of an SLR, but also has most of the features of an SLR along with a larger sensor. The higher priced S100 is an advanced camera with full manual control including more dials and buttons. It does have a larger sensor which will allow for less noise in photos taken in low light, but not a huge difference if your main purpose is online posting for websites. If you have the money and are willing to carry a bigger camera, a dSLR will take much, much better quality photos indoors and out. Plus, they have super fast start up, so you never have to wait for the camera to turn on and the lens to extend.
Most compacts will take excellent photos outdoors, but don't do too well in dim light without a flash.
Here is a link to my Flickr photos with the Canon S95.
http://www.flickr.com/photos/31657193@N04/sets/72157627112031677/
