Sheard has not had a double digit sack season, even as a DE in a 4-3. Jerry Hughes is ascending and plays the money side, RDE. I will take his 10 sacks and better athleticism over Sheard.
While Sheard has yet to record a double-digit sack season...he averaged 55 tackles and 8 sacks as a rookie & 2nd year player at LDE, with not a lot of talent around him (particularly as a rookie). I think Hughes is coming into his own, but it definitely doesn't hurt playing alongside Mario Williams, Dareus, & Kyle Williams. Hughes finished tied for 2nd in sacks on the Bills along with Dareus (10 each). Minus the production from Hughes, the other 3 starting DL players accounted for 30 sacks...that's 2 more than the Cowboys entire 2014 team total of 28 sacks! The Bills led the league with 54 sacks.
To Hughes' credit...he took advantage of 1-on-1 matchups & won more than his share regardless of the opposing blocker. He also produced as more of a situational player in Mike Pettine's 3-4 scheme (same one Rex Ryan runs).
Sheard has played 4-3 LDE & 3-4 ROLB. IMO, he could contribute on either side in Dallas. Both Sheard & Hughes are athletic enough to have played OLB in 3-4 schemes. Hughes is more quick twitch.
I like both Sheard & Hughes...and wouldn't mind adding either. Another plus, is that both are still fairly young. The addition of either one could also potentially create some more flexibility at the DE position.