I've seen these two compared quite a lot in the past few days. They obviously don't have a lot in common aside from the fact that they used violent tactics to overthrow a government and build (or rebuild) a nation, but I thought this would be an interesting match-up.
The main comparison I've heard is about the handing over of power. Mandela, like Washington, was "first in war, first in peace, first in the hearts of his countrymen" but didn't try to take advantage of that to rule, didn't try to set up his own faction with absolute power in the new government. That's all too rare in history. A major difference with Washington is that Mandela's impact was not all that much as a military leader.
Mandela resisted the call for revenge. He's like a Lincoln figure in that way, only imagine if it were Frederick Douglass instead. Washington was more successful, but he had more to work with - no lack of quality comrades, contemporaries and successors who had a healthy tradition of self-governing institutions. On the pro-con list Mandela has got to win, but I feel I know George Washington better and that sentiment is a powerful force. Plus when it comes to Washington we know how it turned out. I think Washington's iconization can obscure his accomplishments as much as his faults - maybe that's true of Mandela as well.