If we're going to insist on taking just one of either Dempsey or Donovan, then I think it's pretty clear which one deserves the nod. It's Landon.
Lebron James once famously said that if he really wanted to, he could win the NBA scoring title every year. (He also said he'd win a million championships with the Heat, but I'll give the King the benefit of the doubt.) The implication was that doing so wouldn't give his team the best chance to win. The King has put his money where his mouth his by providing an average of 7+ assists per game in every season since 2007, and despite his well-publicized struggles in the playoffs, you can't argue with his regular season success.
On the other side, I've been at Madison Square Garden when Carmelo Anthony takes over the fourth quarter of a game, and when he gets going, he doesn't miss. Like, ever. But 'Melo isn't exactly known for his ability to spread the ball around. He's also not in the conversation for "best player in basketball."
Here's my knock on Dempsey: Like 'Melo, the dude doesn't pass. He breaks ankles and scores goals. He scores scrappy goals; see US vs. Panama, 6/22/11. He scores wonder goals; see Fulham vs. Juventus, 3/18/10. But for the US - a team that has been loudly and repeatedly criticized for lacking creativity - that's not enough.
Donovan is a game changer. Period. He does what Dempsey cannot in making everybody around him a better player. And that's the point: the Greats make others around them great.
Like Lebron, Landon has his detractors. Unlike Lebron, he's done enough to shut up his critics. He was the best US player at the 2010 World Cup. He's won in MLS. He's done well in his last two stints abroad. And his ability to pick out a pass separates him from Dempsey in his ability to break down defenses. That makes him the more dangerous and, to me, more valuable player in terms of what a team needs.