1) Has an amazing 90 base power.
2) 20% chance to increase attack by 1 stage which would give the move an even greater power!
1)90 accuracy means it has a little chance of missing,which could be a problem in some stages of the game.
1)Has a 30 % chance to flinch the opponent.
2) Has a higher accuracy than Meteor Mash
1) Has less power than Meteor Mash,which could prove to be crucial
2)It can only flinch if the opponent has not moves yet.Meaning it would not flinch opponents that are faster than Mega Metagross.
3) Cannot flinch Pokemon behind a substitute or Pokemon with inner focus.
I would go with Meteor Mash here because of the power and the chance to increase attack,despite the lack of accuracy.If you are worried about the accuracy,have a another move that has 100% accuracy so that you can use it on the crucial stages.
Hope I helped!