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This is a tough one for me.

Mega T-Tar is essentially just a better T-Tar. He still beats the same Pokes, and still loses to the same Pokes. 100/150 Physical defense sounds awesome. Except you are still 2x weak to Ground and 4x weak to Fighting, Your stats are a little better all-around. Your Speed is still sub-par, your physical Attack can punish even the bulkiest of physical walls but there's two problems. One, you can already beat physical walls with that impressive 95 base Special Attack which gets no boost. The other problem is the loss of Letovers recovery. Seeing as you're still matching Tyranitar up against the same Pokemon, I don't see the purpose of wasting your Mega slot on him unless you just have no other candidates. Even then, it's still personal preference since you do pass up his only recovery method.

Mega Aggron is so ridiculously bulky it's stupid. And losing the Rock typing along with Filter makes it that much better. No more 4x Fight/Ground weaknesses. But can Mega Aggron function as a physical wall when Close Combat and Earthquake still nail him for 1.5x damage? (200% damage but 25% less damage from Filter = 150% damage)

So he can't "wall" exactly, especially since Fighting/Ground are arguably the MOST common physical attack types. Special Defense is slacking too. Fortunately Steel typing gives him plenty of switch-in opportunities. Then, if you can scare the opponent out you can smack the switch-in really hard off that Base 140 Attack.

Another flaw arises, his Base 50 Speed.. You could set up Rock Polish, but I don't see him pulling off many sweeps with Iron Head.. Stone Edge/Earthquake gives good coverage, however. Let's take a quick look at some common Physical Attackers that he walls..
He walls Azumarill, Scizor, Metagross, he does well against defensive Gliscor (most Gliscors), Aegislash w/o Shadow Ball (most actually carry it) and Dragons that don't run Overheat/Special Fire move. Which is actually pretty common.. Donphan, Lucario, and Cokeldurr, Garchomp, Hippowdon, Landorus-T are all common Pokemon you'll run into that have no problem taking down Mega Aggron or at least coming close. Gyarados and Toxicroak also could probably beat him one on one. Gyarados can Mega-Evolve so he now resists your STAB and isn't weak to Stone Edge, but neural to EQ, If he already set up he can spam a couple EQ's. The Bulk Up/Substitute/Drain Punch Toxicroak can handle you with relative ease also.



TL;DR version: Mega Aggron is underwhelming and Mega T-Tar is just T-Tar. My vote goes to Aggron, because regular Aggron sucks and the Mega-version is a huge improvement and at least gives you some different options to work with.

If the vote were regular Tyranitar or Mega Aggron I'd vote for Tyranitar. xD
He will always be an OU staple.

If I had to pick one for mega, probably Aggron. But regular Tyranitar is useful even without the mega evolution. Sand Stream and Stealth Rock in particular are too useful to pass up.