I would probably be a lot more interested in the Brent years if there weren't so many great Keith shows available.
I think that one of the most overlooked element in analyzing the Dead by period or style is the keyboardists. The proto-dead (65-68) is fueled by Pig's often minimalist organ work, a style that often lent itself more to psychedelia than blues. Constanten's work was definately trippy--kind of a churchlike sound that helped float the guitarists along; not a lead so much as a color fill. Still, nice, and it fit the early Dark Star format. Keith--well, now you're into a major change. Obviously jazzier, and the switch is from organ to acoustic piano. The man could play several styles, which was perfect, as the Dead were playing all sorts of stuff. His only negative side was, of course, Donna. The 72-74 Keith-dead and the 76-78 Keith-dead are also different as he changed his textures to meet the new format. Brent--well, Brent seemed to me to be more pop oriented, more into the melody than the previous keyboardists. Like Lon, I just don't like that sound. Sorry!
Bruce added a beauty and texture to the melodics of the tunes and jams. Vince---beats me.
But if you slice up their career, it seems to fall into segments which are governed by the given keyboardists.
One of the things I love about Keith is his big ears. He was always tuned in completely to where the Jerry and band was going, and pushed them on in very complimentary and subtle ways. I haven't heard that many Brent shows. In what I have heard, however, he strikes me as doing mostly his own thing and challenging the band to adjust to him. I also didn't find what he does to be all that interesting.
But part of my preference might be subjective. My last live Dead show was Winterland: 12/31/78.