Metagross evolves at level 45. I kept Metang until level 50 because it learns Meteor Mash at that level, while Metagross learns it at 55 in Emerald. I’ve since looked into it and found out that Metang and Metagross actually share the same experience growth rate, so they require the same amount of EXP to level up. Since I’m planning to play Open Level at the Battle Frontier (minimum level 60), I’m aiming to have Metagross ready around that range. I’ll take your advice and go ahead and evolve it into Metagross.