I think of this for their '70s work (or starting in '69 with "Space Oddity") and go with Bowie. Elton was the king of pop for a couple of years in the middle of the '70s, peaking with Goodbye Yellow Brick Road. But Bowie overall was more consistent, including the surge at the end of the decade with his Eno albums. After the '70s, Bowie and Elton each had their moments.