As with many choices, it's very situational and depends heavily upon personal preference and the rest of your team. Garchomp is a fast, powerful Sweeper with a good movepool. On the other hand, Metagross is bulky, strong, and has great defensive typing. So...
- If you need something to just take out the enemy fast, definitely go with Garchomp.
- If what your team requires is a tank to take hits and dish them out, by all means go with Metagross.