I am doing an upgrade. It's just easy as that. Apple will not be giving it as an option if it will degrade the quality, speed, etc. of Snow Leopard.
I come from a PC background and have been using Macs for 4 years now. Yes, a clean install of Windows can sometimes be better but then I also opted to upgrade to Vista from Windows XP with no problems at all.
I upgraded from Tiger to Leopard and had no problems, compatibilitywise and speedwise.
I can understand if some people here have been installing crap in their Leopard for the past few months and have been getting sloppy Leopard performance. But if you're not experiencing any problems now, why do a clean install?
Listen people, make it an easy and painless decission. Upgrade! If you have problems later on then clean install...which I doubt you will.
Let the upgrade speak for itself and let it run it's course and judge it later.