I doubt the Pistons move either of their young bigs, but if it was gonna happen I would think Monroe, as they have 2 additional years of (cheap) control over Drummond. Monroe is a darn nice player, but 15 and 10 and not a big shot blocker shouldn't be untouchable. Drummond looks like he might be the better of the 2, at a more-premium position.
Now if it was neither, I think about the best the Pistons could do would be:
Rondo, Wallace, Lee
FOR
Charlie V (expiring), Stuckey (expiring), Knight, Kaldwell-Pope, 2017 1st rounder
I don't love any of those pieces, but the combination of cap relief and 3 young players isn't terrible.