I'm guessing when you said mega's that you want a comparison of them not the un-megad forms
Mega Lucario
Pros:
Having Adaptability makes this thing do incredible damage
Great speed (max 355)
Greater Attack (max 427)
And a close Sp. Atk stat (max 416)
Can learn extreme speed
Learns a stab bullet punch
Can be used as support with Heal Pulse
Cons:
It still doesn't have the best of speed and a scarfed attacker will destroy it
Mega Alakazam
Pros:
Massive Sp. Atk (max 493)
Massive speed (max 438)
Could be a good calm mind sweeper
Vast array of hard hitting attacks such as Psychic and Focus blast
Can turn the tables on a defender with guard split
Sweeper Material
Cons:
Ridiculously low defense and special defense
Is destroyed by fake out
Kangaskhan will destroy it
Trace can be good but is mostly bad for you
So in conclusion I would choose alakazam, those high stats are just to good to miss out on and your team needs a psychic.
