I would like to say trade both but I don't think you're going to get what you want for either of them. Span missed a lot time this season (contrary to the recent post) and likely would have minimal trade interest. I'm also not sure that Revere has a lot of value either. It's possible that a team like the braves match up perfectly if Bourn leaves and there could be a pitcher/OF'er swap but I think it's pretty likely that both OF'ers return next season because of a limited trade market.
In an ideal world the Twins would be able to trade both Willingham and span for a decent set of prospects hopefully including pitchers. And since it looks like Willingham is going to finish the season strong I would put him on the block this offseason and next July.