+) sky-high Attack and almost as good Special Attack
+) stellar speed
+) enourmous movepool
+) available relatively early
+) Magic Bounce
+/-) takes up Mega slot
-) shoddy defenses
-) hard to find
Mega Absol is a powerful sweeper with wicked stats, however it can barely take a hit due to 65/60/60 defenses. It has access to a plathora of coverage moves, including Psycho Cut, Superpower, Ice Beam, Fire Blast, Shadow Claw and Rock Slide. It is also available pretty early in the game, however it has a low encounter rate. Choosing Mega Absol would also use up your Mega slot.
+) offensive stats nearly equal to Mega Absol
+) impressive movepool
+) Illusion is fun to use
+/-) doesn't use up you Mega slot
-) defensive stats nearly equal to Mega Absol
-) Illusion can be confusing
-) available very late in the game
Zoroark is basically a slightly weaker Mega Absol that can hold an item and doesn't use your Mega slot. It has a similar movepool, having access to U-Turn, Flamethrower, Foul Play, Shadow Ball, Focus Blast, and its signature move Night Daze. On the dowside, it is very frail and is only available in the very late game, obatainable only on Route 20 and in Pokémon Village.
Zoroark < Mega Absol
While only having slightly better offensive stats and virtually the same defenses, Mega Absol has a slightly wider movepool, better ability, and — most importantly — is available much, much earlier than Zoroark (I haven't played X in a whil, but if I remember correctly, it is only catchable post-game). And frankly, it looks cooler.