You should be asking yourself whether you have any good reason to get either considering you already have two compacts. It almost feels like you're buying another car to throw in your garage for no other reason than because it's new.
Actually it was because I am going to Cape Town (where I have never been before) and wanted to make sure the pictures captured were of excellent quality.
Image quality is relative. It's relative to how large you intend on printing and/or viewing your chosen images. The ST95 you already have doesn't take photos of THAT much worse quality than the WB850, and arguably takes better quality photos than the WB150F. The main thing you'd be gaining out of getting either of the new cameras is a significantly longer zoom range. If anything, you'd be paying through the teeth for a minor upgrade in image quality and more zoom, and end up having three compact cameras that are largely similar.