Well, I really don't see Sam and Jack acting like equals -- They're still on a "sir", "Carter" basis after eight years, even while they're on leave. On the few occasions that Sam's tried to go the 'Jack' route, Jack has completely smacked her down (Fragile Balance and Grace come to mind).

Whereas with Laura and Lee, it seems like there's this mutual level of respect and trust. They both care deeply about the well-being of the people of the fleet. They're both big-picture people. I can see Lee becoming the President, some day (after Roslin dies... *is sad*), because I think that he has the integrity and the ruthlessness needed.