Danny wasn't a singer, or a songwriter, while Mark is both. Being a guitarist, a singer, and a songwriter makes one a more well-rounded musician, but perhaps Danny focusing on "merely" playing guitar was instrumental (hah!) in him becoming so accomplished at it.
The matter of technique is very misunderstood, even amongst musicians. The assumption is that the greater the technical ability---"chops", the better the music. It just ain't so. Without fail, every time I meet a new person of median age, and they hear I play drums, they bring up Neil Peart. Neil worked long and hard at developing his chops, yet when he assembled a distinguished group of musicians and recorded a tribute to Buddy Rich, he displayed, to his embarrassment, that he was incapable of "swinging"---playing a shuffle. How lame is that? It's only the foundation for all of Blues, most of Country, and a lot of Rock 'n' Roll!
I tell the following story---told to me by Evan Johns (who worked with Danny Gatton as a singer, songwriter, and second guitarist) every time the matter of technique comes up amongst a gathering of drummers: Danny Gatton had an ever-changing group of back-up musicians, and on one particular night had a drummer playing with him for the first time. After the first set, Danny said to the drummer: "You know all that fancy s*#t you're playing? Don't".
Danny had not only technical chops, but taste---musicality. It's hard to define, but it's what separates the men from the boys. A major element in taste is knowing what NOT to play. There is a famous quote, attributed to Duke Ellington, Count Basie, and Miles Davis: "It's not the notes you play, it's the notes you don't play". That's what it's all about. Danny knew which notes to play and which to not play, and so does Mark. Jerry Garcia, not so much ;-).
