So, for these Pokemon's typings, Metagross is a Steel and Psychic type, leaving it weak to Fire, Ground, Ghost, and dark type attacks. This Pokemon also has an immunity to poison type attacks. This Pokemon resists Normal, Ice, Grass, Psychic, Flying, Steel, Fairy, Dragon, and Rock. A very defensive type combo. Salamence is a dragon and flying type, leaving it weak to dragon, fairy, and rock type attacks, with a 4 x weakness to ice type attacks. This Pokemon is immune to Ground type attacks and resists Grass, fire, water, fighting, and bug type attacks. Another solid type combo.
However, their is always a winner and the winner of typings goes to of course Metagross in both formats. Metagross just resists more and doesn't have a 4 x weakness to anything. It may be weak to a lot of common competitive types, but it is still good enough to resist as much damage as possible Ice may not be a common type in the competitive scene, but a lot of Pokemon carry ice beam in singles and blizzard in doubles. Both of these are things that Salamence just can't outlive.
Stats and Possible Abilities:
Special Attack: 110
Special Defense: 80
Abilities: Intimidate (lower the oppoosing Pokemon's attacks on switch), Moxie (raises the users attack by one stage if opposing Pokemon is KO)
Special Attack: 95
Special Defense: 90
Abilities: Clear Body (prevent the other Pokemon from lowering its stats), Light Metal (halves the Pokemon's weight in battle)
The winner for stats and abilities is Salmence because of a few things. One, its abilities are good in each format, intimidate for doubles and moxie for singles. Also, it just has better stat distribution and can go as a mixed attacker.
My computer that I am typing this on is lagging, so I will leave the rest in another post.