Honedge is a very strong Pokemon when it evolves (dusk stone found in Laverre),
and it's found really early in the game. It's typing and level-up movepool are good in a lot of the gyms. It will be great in the Rock, Fighting, Fairy, Psychic, and Ice type gyms. Sacred Sword, King's Shield, and Iron Head are all great moves, and it's TM movepool is decent as well; including Rock Slide, Shadow Ball, Flash Cannon, and Shadow Claw. The only downsides to Aegislash are it's frailty in blade form and limited HM usefulness, as it only gets Cut.
Mawile, on the other hand is available shortly after the second gym, and while it has a very good typing, and strong moves, it's stats are lackluster without it's mega stone, which cannot be obtained until after the main story. It has many of the same advantages that Aegislash would, like being good in the fighting gym if you can get a level 15 one that still has Play Rough, resisting the Grass gym, and being super effective against the Fairy and Ice gyms, but is still outclassed. It's stats pre-mega really let it down, and it's HM use is limited to Strength, so it really doesn't have any advantages over Aegislash or another Fairy type.
I'd say use Honedge, as it's usefulness starts earlier in the game, and it's a much better Pokemon than non-mega Mawile. Either could work well in the story, however, and if you really like mawile, then by all means use it.