Garchomp has a pretty good HP stat (108) and attack (130). However, Metagross's attack is slightly higher (135). But it's hp has a -28 difference (80).
Being a steel type, Metagross's defense is an obvious 130, and is 140 if it's a bold pokemon. However, it's attack is lowered to a 125. But you can improve it.
Garchomp, on the other hand, has a 95 defense stat, which is really good. If it has a bold nature, again, it becomes 105. Same with lax. The Lax Nature has its flaws though. The sp.def is lowered to a rather pitiful 75.
Metagross's sp.def is a ninety. 100 if it's a careful or calm pokemon.
Garchomp's speed is is a 102 compared to Metagross's 70. However, if it has a hasty nature, it'll become an 80. Still, you have to improve this stat. Metagross is pretty bulky, so you should help its speed stat.
Garchomp's sp atk is a decent 80. If it has a mild nature, it's a 90. Otherwise, you should improve it.
Metagross's is an excellent 95, and is 105 if it has a modest or mild nature.
Overall, they both add up to 600, being pseudo legendaries.
Metagross's learnset is great, but it can't learn a very wide range of TMs and HMs. Garchomp, on the other hand, has a fairly decent learnset, and it has a WIDE range of TMs and HMs.
Metagross is great, both offensively and defensively. Also, thanks to steel-and-psychic combination, it's weak to a very few types. Psychic, which is weak to Bug, is saved by the Steel type, which is of course resistant to it. Same with Fighting. And Clear Body prevents stat reduction.
Garchomp is pretty fast, good defensively and offensively. However, the Ice type is something you should fear. Unless your defensive stats are ridiculously high, you'll probably be 2 or 1 KO'd. Also, water and grass can be a bit of a nuisance, thanks to the Ground type.
Sand Veil, during a sandstorm, raises Garchomp's evasion. However, it's stats can be reduced, unlike Metagross.
Even though Garchomp is fast AND good at defending and attacking, it's weak to a fair amount of types, due to it's ground type. Also, it learns a wide amount of TMs and HMs. Metagross has Clear Body, is weak to a few types, is great both defensively and offensively, AND it has a great learnset when leveling up.
Overall, Metagross is the best simply, thanks to types and good stats. Garchomp is a good choice but I HIGHLY recommend Metagross.
(oh come on, I have a haunter and a roserade on my team. you don't need to worry about the two same types.)