To me I don't really know exactly who the Wizards would have to trade if one of them were to go.
In any circumstance, either of these players are tradeable, should a suitable opportunity present itself, in the words of Ernie Grunfeld. In fact, every player is up for trade except John Wall.
But if one were to be traded, I'd rather trade Blatche. There are a number of players who are taller than 6'9" who have decent midrange shots, or better and can drop 20 any night, like Blatche. He also has a sizeable contract that we could get two younger players back with, or possibly a player plus a pick.
McGee also has a midrange shot (though he doesn't use it much in games), and of course his athleticism is crazy. He still needs to tune up his fundamental skills, but these can be taught, and he is trying. Given his background as the son of one of women's basketball best ever stars and one of the pioneers of the WNBA, he certainly has picked up much of his mom's work ethic. Also, he'd only be a rookie right now if he played through his last two years of college and you can argue that perhaps he may be a more developed player if he stayed in college and got to play starting minutes at Nevada for two years rather than be a bench player in the NBA under a combination of Brendan Haywood, Darius Songaila, and Etan Thomas who ate up a lot of his minutes in his first two years.