In my opinion, both mega evolutions would do for their respective tier. Mega Tyranitar gets a substantial boost in attack and defense, therefore making it an effective bulky attacker for the OU format.
If you're going with the Ubers format, Mega Metagross is also a great option. Metagross has lackluster special defense, so it isn't an effective bulky sweeper, nor an effective fast sweeper with a base speed stat of 70. However, when it mega evolves, it can out speed a large number of opponents. If an opponent happens to be faster, than Mega Metagross can take hits. Lastly, upon mega evolving, Metagross gains "Tough Claws", boosting most if its physical attacks.
It really depends on the format. But if I had to choose one, it would be Mega Metagross, because regular Metagross isn't a top tier Pokémon(UU), while Tyranitar works almost just as well its mega evolution.
***btw I'm not exactly a master in competitive, so please correct me if you think my opinion is totally wrong. :)