I think you gotta be more specific, there's the Great Society till Volunteers Grace and there's the 70's Grace. In my opinion the great one was the early Grace, something happened to her after woodstock and never got the same sound again. Sandy is amazing always, on studio and live (at least at the "Live At The BBC" albums I've heard her perform). If we're talking about powerful voices, only Signe Anderson could beat Grace, no one else could belt like them, Jefferson Airplane had the most powerful singers. Sandy is always softer but firm, like on fotheringay or who knows where the time goes. 70's gracie sometimes sings out of tune and there ain't a live performance I thought was GREAT like the early live performances, like Monterey or Woodstock or Bless Its Pointed Little head. Sandy Denny is like Karen Carpenter, I think they didn't have the hability to sing out of tune even if their lifes depended on it.
If we're talking about leadership, Sandy wins. She joined Fairport after Judy Dyble (my favourite singer) was kicked out. Early Dyble-Fairport was playing the Frisco sound, they were called "The british Jefferson Airplane" but as soon as Denny came in, she changed it all and went folk. If you listen to fairport's first album (Fairport Convention) and then the second (What We Did In Our Hollidays) you'd think it's two completely different bands. No psychedelic rock, only british folk. Fairport kicked out the male singer (the equivalent of Marty Balin, Iain Matthews) and she was the solely frontman and songwriter and Lead vocalist on almost every song. Grace only got this kind of attention while on The Great Society but on Jefferson Airplane she was reduced to two songs per album and from Blows Against The Empire on (Bark, Sunfighter, Long John Jefferson Starship, bla bla bla), her vocals fainted altough she was featured prominently and after Red Octopus, she did nothing decent. But if we take both singer's two best songs (White Rabbit and Somebody To Love vs Who Knows Where The Time Goes and Fotheringay) Gracie wins by far.