Statistically, during their peak periods, it's not even a close comparison, Cairns dominates Flintoff.
Cairns: From 1999 to 2004...
Averaged 45.6 with the bat, with notable performances against Australia, South Africa, the Windies and cameos against England.
Averaged 25.69 with the ball, with notable performances against England, South Africa and the Windies.
Windies weren't at their best, but were still a reasonable force with some good players in that period.
He didn't play as many tests as he should have by the end, but he was a world class all rounder for the last stanza of his career and had he been Australian or English, would probably have been hailed as one of the great all rounders of the game.