You have 2 special attacker and 3 physical attackers.
You don't have psychic-types and bug, however you have a Steel-type.
Mega Alakazam
Pros:
Its SpA (175) and Speed (150) stats.
It includes powerful moves such as: Dazzling Gleam, Energy Ball and Focus Blast.
Trace ability can be good and bad. Depends on what ability it copies.
Cons:
Trace ability can be good and bad. Depends on what ability it copies.
It doesnt have dual type, meaning it only gets STAB from 1 type.
Alakazam @ Alakazite
Trait: Trace
Nature: Modest
EVs: 252 SpA / 252 Spe / 4 SpD
Shadow Ball
Psychic
Dazzling Gleam
Recover / Energy Ball
Mega Scizor
Pros:
Has access to priority Bullet punch.
Can sweep teams after using swords dance a couple of times.
Great defensive typing.
Technician Ability to power up bullet punch.
Learns roost for great recovery.
Cons:
It will be 2ko'ed by any good fire type move.
Slow and unable to do a no bullet punch sweep against opponents without spamming agility
can be stopped by an encore user or a steel resistor such as the fire types.
Scizor @Scizorite
Trait: Techinician
Nature: Adamant
EVs: 252 Atk / 252 Def / 4 HP
- Bullet Punch
- Swords Dance
- Bug Bite
- Aerial Ace / Brick Break
Conclusion
You should pick one that can be very usefull in your team. Attack and Special Attackers can make the difference. Also Scizor having great moveset with priority + Swords Dance, makes him a killing machine. High defenses.
Mega Alakazam with very great Speed and Special Attacker, outspeeding everything, but the ability.. yeah.
I would go for Mega Scizor!
Swords Dance + Bullet Punch + STAB + Techinician + Priority + Great Attack Stat.
Hope this helps.