Cloning just takes the image as it is now and has it saved up for that rainy day. Imaging does the same thing. So really what you want to do is backup with increment updates. That way when you make a change or get a update that will be added to teh backup. This way you wont have a image from 9/30/07 and then your PC crashes in Feb and you have to resort back to Sept.
I only suggest this option if you do make many changes and file changes. Liek word documents.
Also stom software like Acronis backs up the whole drive that the OS is installed on. So if you have it installed on a 320 GB drive it will want to back all that up. So a 250GB external wouldnt work.