Both can be impact players. All would depend on FA.
I think if you are fairly conservative, you take Green. He is 6'4" 325lbs and athletic. He moves people when they don't want to be moved. He has the smarts and athleticism to be like Martin and be there for 10 years.
Dean on the other hand has a higher upside in my opinion, but also more likely to bust.
I have watched Texas A&M for many years now and Green is a plug in starter who can sub up and down the line as needed. If Dallas picks him AWESOME.
With that said, I think I would pick Dean. Dean can lift the level of the D play by quite a bit. Parson, Cox, Dean allow you to be dynamic on D and let's Parsons move as needed to create a nasty matchup nightmare.
Steele was a UDFA -- and many want him to start. LCollins played LG very well a while back. Alarcon is a huge guy who might very well come around. McGovern, Biadasz, Farinok all are young and might be able to handle starting with a correct offseason. Draft picks from 4th down can be picked for OL.
IMO -- the easiest way to make Dallas very good is get a legit 1T, MLB, and TE in the first three rounds (order dependent on who falls) if they don't lose too much talent to FA. Then stock up on OL to develop.
1) NDean - LB Georgia
2) TJones - DL UConn
3) JRuckert - TE Ohio State
Then OC, OG, OT for the next rounds with (a S or second legit TE) would be very good.