I believe Jolly is the better nature here. For one, Metagross already has a monstrous base Attack Stat of 145 which is good. Now, Jolly can serve some really important purposes. For one, it helps tie or beat many Pokémon like Mega Latios, Lugia and others. Also, Jolly can outspeed anyone who decides to put a non-Speed boosting nature on their up to a 125 base stat. Some of these are Mega Lucario, Mega Salamence, Arceus, etc. However, if you run Adamant, you are outsped by speed boosting natures on Pokemon like Mega Blaziken, Genesect, Mega Kangashtan, among others. Because of the possibility of outspeeding Pokémon like those and definitely outspeeding others, I believe Jolly is best.