"Arguments that there is something mysteriously dangerous or deficient about the default upgrade procedure — and that you should do a clean install instead, followed by tedious hours manually migrating software and data and preferences from your old installation — are voodoo. Apple’s installer engineers spend a ton of time making the default upgrade procedure as convenient as possible.

If you’re not a typical user; like, say, if you’ve been running pre-release developer seeds of Leopard, or if you’ve diddled with your 10.4 system software in unholy ways and really would like a factory-fresh start with 10.5, then I recommend the Archive and Install option. (That’s what I do, when upgrading from previous developer seeds.)"


http://daringfireball.net/2007/10/murphys_law