Playing late in the round, isn't that much of a disadvantage, given the rolling lockouts. Trade to someone in that same team, but being able to undo your change, or trade to multiple teams not yet played would be the only disadvantage IMO.
The advantage could be that you had a low score from a single game week player, and you need to better that, but this could be applied in any position really, but the flex allows for all positions.
The only thing I feel may be a hinderence on the flex is choosing a dbl game week player, especially if they may not play the second game. So, I would choose a SGW player only.
I personally am going Short. If for some reason, he doesn't play, I will move to Overton who I don't have.