Usually when a QB regresses especially after a serious injury they never return to form. RG3 has been given every opportunity to regain the form he showed as a rookie and has never looked the same. He can't run the way he use to and his career has gone in the toilet. I'm not high on Manziel and his off the field antics and maturity are a real concern but he's at least shown some development on the field from year one to year two unlike RG3 who continued to regress after year one.
Honestly, I think Manziel has the ability to be a terrific QB. The circus he would bring to Dallas with him- makes a bad fit. I regret the team passing on Manziel about as evenly as I'm glad they took Martin. Had Johnny F. been in the right situation with leaders like Romo and Witten to show him the ropes and call him on his off the field bs., I think they would have their QB when Romo steps down.
Now it's too late, despite the dropping price tag on Manziel. His presence coming off a likely 5 win season would just create too much havoc for the front office and fodder for opponents to tee off on Cowboys fans everywhere. Following football is already difficult enough for Cowboys fans, and the players begin their careers as targets as it is. Find someone else.