I think Leno has a better floor, in that Martinez still makes errors, but Martinez to me has a higher ceiling due to his all around play (kicking, ability in the air, etc.). Also, I've never got the feeling that Leno was in sync with his CB's, seems he's more of a shot stopper than a marshall.
For me, I think you have to let Martinez start the season, but Arteta should be honest with him and let him know Leno will be the (long term) #1 if he doesn't maintain his form. From there Martinez can decide if Cup games are enough, or if he wants to move on.