yeah, this has been discussed ad nauseum all spring, all offseason, and now all fall camp, but why not...
What I recall reading is that QB coach verduzco has some actual metrics that he uses, including something like a 3 hour test that they take before fall ball and at the end of fall ball to test their knowledge of the offense, defensive alignments, where to go with the ball in those situations, etc; plus other stuff like charting drives in scrimmages, charting turnovers, production, etc in practice.
anyway, the general fan/media consensus is that gebbia is the better thrower and a film/study junkie, while Martinez is the better runner and better overall athlete (and is already bigger and probably stronger)
SF has stated that the QB doesn't necessarily need to be a huge running threat or have 4.4 speed but instead someone who can pick up 4 yards or so when the situation/play calls for it, but more importantly, the QB has to make the correct decisions on the field, especially since they run a lot of RPOs (run/pass options) but also on the zone read, traditional/nontraditional option runs, and with multiple route combinations in the passing game.
for me, I think that Gebbia's floor is higher (i.e. he'll make less mistakes) but that Martinez' ceiling is higher (he's capable of more)
I also think it would be tough to completely block out the thinking that while yes everyone wants to win right now, if they have a long leash and Martinez is the QB of the future, why not play him more now?
for me, none of us have watched the practices to have any sort of a clue as to who performs better on a daily basis, who knows the offense better, who makes the proper decisions the most often, etc.
but, I think that Martinez being better in the running game simply opens up more of the offense compared to Gebbia. Gebbia can run all of the same plays, but would be a decoy more often than not, whereas I think Martinez is much more of a running threat that would cause problems for defenses and open up the other players to have more success. I think that the offense and the playcalling is designed in such a way as NU doesn't have to have a super accurate passer, and I think that Martinez ability to possibly extend plays with his legs will be a big difference as well.
also, seeing photos of the QBs in fall ball, Gebbia still seems damn skinny and seems to need to wind up his entire body to throw the ball while I think Martinez is already bigger and more put together and able to throw the ball more easily and take punishment better.
either way, I think we'll see some mistakes since neither one has played a meaningful snap in over a year aside from a spring game or two. what I think another difference might be, though, is that Martinez is confident in his ability to run, so when something isn't open, instead of throwing it away, he might be able to roll out and either pick up a few yards or hit a pass on a broken play that otherwise wouldn't have happened.