The only reason DD is doing well at the moment is due to his size as a SG.
If we move DD to SF, we are taking away DD's only advantage. He is simply not strong enough to guard average starting SFs from other teams. DD playing for starting SF would be as bad as Barg playing for starting C. Yes, it is THAT bad.
Having said that, Ross-DD lineup will be a huge disaster. If we want to develop Ross as our starting SG, we definitely need to trade DD. If DD is considered as poor man's Vince Carter, then Ross is going to be like T-Mac. Remember how much we regret to lose T-mac and watch his god-like plays? Same will happen to Ross, if we keep DD on a roster.
Here's direct comparison between DD and TR.
Hardware(height/weight): DD > TR
Foul drawing: DD > TR
Athleticism: TR = DD
Rebound: TR > DD
Defense: TR > DD
3Pt: TR > DD
Looking at Ross, he is capable of delivering everything DD is delivering, except attacking rim for foul drawing. This being said keeping DD will only postpone Ross' development.
I would say, we should trade DD when his value is up high and get a decent SF back to pair with Ross.