An important point to bring up is if we really wish to have him as our quarterback for the next several years. Many Husker fans dislike Martinez and his inability to develop into a complete passer. He can run, but that doesn't seem to be the main priority in these fans' view.
Fuller is a different runner than Martinez but he still is, at the end of the day, a runner and not a thrower. If people don't want Martinez now, they'd more than likely not like Fuller for the future. Of course they don't see that now because of Devin's ranking but it is the truth.
A move to a more traditional quarterback, or at least one who has the adequate ability to throw the ball, seems like the most desirable movement, and that would involve Carnes or Armstrong, not Fuller.
Are Carnes' and Armstrong's passing skills really head and shoulders above Fuller's though? I don't see a huge difference.
I think as of right now, Fuller's passing ability is just a thread behind Armstrong's, though a case could be made that it is by more than a thread. What really seperates Carnes, and more specifically Armstrong, as throwers in comparison to Fuller is that their potential as a passing quarterback is far greater.
Fuller shows the ability to pass, but he is, and always will be, a run first quarterback, and while that may not seem like a bad thing given his athletic prowess, it severly limits his potential to become a whole quarterback. His highlight video doesn't show him making college level progressions and reads - if his targeted receiver is covered, he takes off running rather than scanning the field. Also a point that has not been mentioned much is that his body frame is delicate and I really doubt he and his style could take a full year's worth of punishment in the Big Ten without getting hurt.
Armstrong on the other hand, is a pass first quarterback. Unlike Fuller, all of the schools recruiting him are doing so as a Quarterback and not as an athlete (while this speaks volumes of Fuller's athletic prowess, it also speak volumes on the schools' lack of trust in his development as a quarterback). In his highlight video there is very little footage on his intermediate game, which is a little alarming, but he shows numerous times (whenever he's not chucking the ball 70 yards) that he is able to scan the defense and progress through his options.
In sum, Devin's mindset of running first limits his ability to truly mature and he will be stuck in the same mold as Martinez and Robinson. His absolute highest ceiling is a considerably less athletic Michael Vick. I don't think Husker fans are looking to have a sequal in the Martinez expirement.
Armstrong could very well flop as his intermediate game is a question mark, but he at least has the potential to become a whole quarterback.
Of course an argument could be made that Fuller is the better player, but let's try to look on the bright side in case we miss out on him, shall we?