No doubt they are both good. ACDC may have more credentials simply because they have been there longer. Slash is becoming a legend though (he's got his own star in Hollywood mind you). In isolation, the guitar playing of Slash has just so much more content than the Young brothers combined. I don't think Michael Jackson, being a perfectionist, would even bat an eye on Angus but then the latter probably would not take the offer anyway.
Slash is just so much more melodic. Give in to Me, November Rain, Estranged, Dont Cry and even the simplistic Double Talkin' Jive has just so much more in them. You can't blame Angus' reliance on the pentatonic scale because in better hands the scale has so much more content (For the Love of God by Steve Vai comes to mind).