cb4life_30,
That's a question that I've asked myself but its not easy to answer. Both the SuperTool 300 and PowerAssist are in the same price range but each has different benefits and drawbacks. Personally, I think that they are BOTH worth having around (the same goes for a few other multi-tools as well).
Pliers: The pliers on the Super Tool 300 are great but it's hard to beat the compound leverage of the SOG PowerAssist. I also prefer the fit and finish of the SOG pliers.
Wire cutters: I've been applauding the SOG cutters since the PowerLock 2.0 was released. The SOG cutter works just about as well as my Klein stranded/solid wire cutters. BUT Leatherman uses 154CM steel and a design that's replaceable/user serviceable. Both cutters are also designed to work a bit differently--the SOG cutter comes to a sharp point whereas the Super Tool has a square edge. At first I didn't think the Super Tool 300 would perform well but I was impressed by how well they work. The SOG cutters do cut a bit cleaner but the thick edge of the Super Tool 300 is less likely to be damaged... And if it ever gets damaged then you could easily sharpen or replace it yourself.
Drivers: SOG uses a #1 Phillips whereas Leatherman uses some hybrid #1/#2 Phillips that seems to fit everything really well. Both work fine but the Phillips on the Leatherman is longer and a bit more versatile. Although I really like the flat head driver on the tip of the file on the PowerAssist, it's other drivers are quite short. Although they're adequate, Leatherman gets the nod for it's longer and hollow ground flat drivers.
Blades: It's definitely not fair to compare the two but I prefer the blade geometry and the accessibility of the blades on the PowerAssist. It may be more fair to compare it with the PowerLock. Although, I don't like the bead blast finish of the blades on Leatherman multi-tools, I do like the larger blade (in comparison to the PowerLock) on the Super Tool 300 . Also its more easily accessible than the blade on the PowerLock.