Alright so first off lets take a look at their stats
Magnezone - 70/70/115/130/90/60 (HP, Atk, Def, SpA, SpD, Spe)
Metagross - 80/135/130/95/90/70 (Same format)
So Magnezone has higher Special Attack, Metagross has higher attack, and they both have fairly similar bulk, though Metagross's is higher especially in defense, and has a higher speed by 10. Metagross in the end, has the higher BST at 600 (Pseudo legendary!) than Magnezone at 535.
Typing. Magnezone has an Electric/Steel typing giving it a resistance to a whopping 12 types, 1 immunity, 3 weaknesses and 1 normally effective
Metagross is Psychic/Steel, with only 2 weaknesses and an impressive 8 resistances, 1 immunity and 6 are normal damage.
Metagross's abilities are Clear Body and Light Metal, with Clear Body being the better one of the two, preventing stat drops from opponents. Magnezone on the other hand, has Magnet Pull which is an awesome ability, trapping Steel types... that is in competitive it's awesome :P. But since this is ingame and the chances of the opponent switching out are minimal, it becomes rather redundant, while the prevelance of random moves which decrease stats increases ingame, so Clear Body is superior there.
Movepool wise, Metagross gets access to a host of different moves like Hammer Arm, Earthquake, Elemental punches, priority in the forme of Bullet Punch, as well as special attacks like Psychic as well as a nice speed boosted in Agility. Magnezone on the other hand, has a movepool dominated by Steel, Electric and Normal type attacks, leaving it walled without a favourable hidden power, which isn't all that likely ingame, and can't increase it's stats much outside of Charge Beam.
Metagross required 1.25 mill exp to get to level 100 while Magnezone required 1mill. Quite a bit less on Magnezone's side, so it would be faster to train if you wanted to rush to level 100 or something.
So yes, Metagross. It's decent mixed offenses allow it to work better ingame, as it is less likely you'll be EV training and everything, so Metagross can use physical attacks extremely well, and Special Attacks to a decent level, compared to Magnezone which can only fire off attacks effecitvely on the special side. Also Metagross's diverse movepool outclasses Magnezone's making it easier to fit onto a team and use. Even though it requires more time to train than Magnezone to get to the same level, it's higher stats and better movepool more than makes up for it.
Sorry for the long answer I had to analyse stuff :3 lol