Well it all depends on what you want in your team bulky defender or a bulky potential sweeper
Mega Aggron
Pros:
Incredible defenses ( max 614)
Access to Iron defense to double his defense
Massive Attack (max 416)
Ability filter to make this capable of taking super effective hits like a sponge
Powerful STAB moves like iron tail and Heavy slam
Can set up stealth rocks
Can use the Rest-Sleep talk strategy well
Can use Roar to slowly faint opponents with stealth rock
Cons:
Not so Amazing special defense
Can be destroyed by encore users
Terrible speed stats
Can be destroyed by taunt but this is dependent on the set you run
Mega Scizor
Pros:
Has access to priority Bullet punch
Can sweep teams after using swords dance a couple of times
Great defensive typing
Better Sp. def than Mega Aggron
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
Works better in my experience using Scizor with a life orb or leftovers than Mega-Evolving it
Personally I think Aggron is much better as a mega than Scizor. As I said before it might be better to run leftovers or life orb on Scizor than to mega-evolve it. But it would still depend on what you want. Sweeper or staller?
Ps. beware prankster if you use these they will destroy both of theese
