I've seen a lot of posts on how to do a clean install off of a DVD or USB disk. I will be upgrading from Snow Leopard. What is the benefit of doing a clean install using the USB method? Ideally I want to erase my entire machine with Snow Leopard and install Mountain Lion on it. If I go to the App Store and download ML, will there be an option for me to erase my entire machine and install Snow Leopard? Or does the App Store approach simply install ML on top of Snow Leopard leaving behind Snow Leopard remnants? So two questions: 1) If I download ML will Apple prompt me with the option of erasing Snow Leopard and starting fresh? 2) If so, is the USB/DVD stick method better than than the option mentioned in question 1? THANKS!