I am going to say Phish > The Dead
I absolutely love both bands, just saying.
Although you really cant compare the two, I will attempt.
Trey can play Jerry, but Jerry can't play Trey. The things that Trey does on a guitar requires much more skill (not taking anything away from Jerry) than it does to play a Dead tune. For example, the Phish song Stash. Jerry couldn't play it. Another reason is the composition of the tunes. The majority of The Dead's tunes follow a simple progression and structure, whereas a good amount of Phish tunes follow no structure at all (ex. You Enjoy Myself). Another being that Phish has consistently had the same lineup (with the exception of the first 2 years before Page came along) throughout their 29 (entering their 30th) years as a band. The Dead has had 10+ different members throughout the years.