The advantage of reformatting is that you set your system up in the cleanest state possible, but that is merely insurance against glitches that may or may not ever occur. There is a big difference in how much work is involved in these two methods of installation, however. So, unless you have had trouble with your system, Ruthless, I don't see why you can't simply do an update. Also, if you do have trouble with Panther for some reason, you will still have the option of reformatting and installing it new later.