Dimebag Darrel vs. Kurt Cobain:
Innovation: Dimebag. Cobain innovated nothing related to guitarplaying whereas Dimebag's whammy work and use of harmonics was innovative to metal and rock as a whole.
Respect from other guitarists: Dimebag. You hear people praise Cobain and Nirvana, but not so much in the guitar community and in any case I've come across, never because of the guitarplaying. Dimebag was well respected. Even when people didn't care for Pantera's music, most would agree that his playing was good. Numerous bands and guitarists have dedicated songs, even made original tracks, in honor of him, aswell as covering Pantera.
Influence: Dimebag's very influential within metal, having influence on old-timers and contemporaries like Kerry King, Scott Ian, Zakk Wylde and Dino Cazares, aswell as newer guys from bands like Lamb of God, Trivium and Killswitch Engage (nevermind your opinion of the bands). As for styles, he was instrumental in the creation of groove metal as a subgenre of thrash aswell as influencing nu metal and the newer style of metalcore. Cobain took the style The Pixies had already created, with no noticeable differences in terms of guitarplaying as far as I know, and made it reach the mainstream. If there hadn't been countless contemporaries directly influenced by The Pixies and Cobain himself clearly stating where he was influenced, he might've had a stronger case in this criteria, but even so, I'd say Dimebag's influence as a creator trumphs that of Cobain as a mediator to the mainstream.
Legendary status: As a rock musician, Cobain. As a rock guitarist, Dimebag. Due to reasons already pointed out.
Importance in shaping the world of rock guitar: As already pointed out, rock guitar hasn't been critically influenced by Cobain. If he hadn't come around, The Pixies still would have, and apart from the slow verse/heavy chorus dynamic between guitar/drums/vocals (which they weren't even the first ones to do, but have been credited with, as far as I know) I don't exactly what you'd point at with Cobain that makes him a quintessential guitarist to rock history. Without Dimebag's playing, chances are entire sub-genres would be different today.
Playing abilities: Dimebag. If he doesn't take all those sub categories of this criteria, he definately takes most of them and some by a lot. Technically there's no question, while not being a terribly versatile guitarist I'd say he even takes Cobain there. Improvisation, something neither do a whole lot of to my recollection, though Dime sometimes just had to "let it rip". Creativity to Dime, through anything from his riffing employing bends, slides, harmonics, tremolo picking, blablablab, to his soloing with the distinctive wails. Cobain had the creativity to.. play power chords with very little, as far as I know, thought as to making the notes ring in a distinctive way by employing varied techniques or whatnot. Live I guess would be a tie, seeing as both have stellar performances and showed enthusiasm in their playing, but whereas Cobain's guitarwork could be played by a one-armed monkey, there are guys like the Licklibrary dude who still can't figure out some of the finer touches of Dimebag's solos (and even riffs, but that was just a sad instructional). Dimebag's guitarplaying and especially his tone is quite distinctive and I guess you could call it original (I haven't heard any other guitarist use such a gainy tone as Dime has on Far Beyond and Great Souther Trendkill prior to those). Off the top of my head, I couldn't even begin to describe Cobain's guitarplaying or tone apart from it being simple and fitting it's purpose, but nothing more, cause it's not distinctive enough.
So that's my semi-biased take on a Dimebag/Cobain breakdown. So I'd like to see where you disagree seeing as Cobain is miles ahead right now.