They're different. Slash is all rock'n'roll while Mayer is more blues and elements of Jazz. Slash's style is way more in your face while Mayer is a little more nifty with his. Mayer probably has a more well-balanced education on theory/improvision while Slash is all feel.

I will say though that Slash has many classic guitar solos that are staples and every rock guitar player will learn at some point, while Mayer has not one. Slash also has developed a clear defined style and tone that he has claim to, while Mayer doesn't really have one. IMO