First, let's look at both megas.
Mega Aggron gets a boost in attack, defense and special defense while Mega Tyranitar gets a boost in attack, defense and speed.
This means that Mega Aggron is meant to be a wall and Mega Tyranitar more of a sweeper.
Now, Mega Aggros gets the ability Filter which reduces the damage takes from super effective attacks by 25%.
Mega tyranitar gets Sand Stream. That summons a Sandstorm, which raises its special defense by 50%, and that's amazing for Tyranitar.
Both Pokemon get an ability that helps them survive.
One bad thing about Sand Stream is that if mega Tyranitar gets KO'd, any non-rock and non-steel types will be damaged by the Sandstorm until it subsides.
However, Sandstorm will help you a lot if you have a Pokemon with Sand Force, Sand Rush or Sand Veil.
Mega Aggron becomes a pure Steel type, so it only has 3 weakness, 10 resistances and 1 immunity.
Mega Tyranitar remains Rock/Dark type, and has 5 weakness,6 resistances and 1 immunity.
So, generally, Mega Aggron has better chances of surviving than Mega Tyranitar.
Mega Tyranitar's base Attack stat is 164, which means it's meant to be a sweeper.
Mega Aggron's base Attack stat is 140, and that also means it's capable of sweeping.
So, my choice is Mega Aggron because it has a better chance of surviving, and can also KO some Pokemon, but it's not better than Tyranitar.
Hope I helped :D