They both had a unique style and sound. I would say Randy was a better guitarist technically, but I think Dime probably had a more unique sound. Me and my wife were talking about him last night(Dime). About how you could tell his tone and playing immediately. Randy also had this, but he came at a time when there were other guitar heros. He stood above many of them back then and his playing was great. Too bad we didn't get to hear more of it.
Dime came at a time when the guitar hero was all but dying or dead already. He really revitalized the metal scene, at least here in the states. If you ask somebody who started playing guitar in the 80's, I'm sure they would name Rhodes as an influence. Anyone from the 90's and it would be Dimebag.
Bottom line, both great and influential guitarists. Different times and totally different styles. I can't choose between them, I think they were both awesome. The bad part is the word "WERE". It's a total shame both these guys lives were cut short because of some stupid acts by others.