Plant, without a doubt. He has the passion that Clapton has always lacked, and can still deliver the goods.
Clapton is a mediocre singer, and these days a passe guitarist. The last good thing Clapton did was the 'Beano' album with the Bluesbreakers. Too many fan-boys worship EC, (as they know no better) whereas RP just gets on with it.
A pair of ears that work.