Each was the right captain for their era (both in-universe and the era in which the series aired). Kirk was a cowboy, a maverick, who used his head but trusted his gut. Picard was a diplomat, collaborative, used his head and followed his principles. If Picard were a captain in Kirk's era, he would have ended up as a starbase commodore or as a diplomat. If Kirk were a captain in Picard's era, he would have been relieved of command many times over.