For the most part, it really does depend on your team, as both these Pokémon are better suited for very different roles, but here's a comparison anyway:
If you're looking for a sweeper, then go Garchomp. There is no competition here. Sure, Crobat may be a lot faster, but at 102 base speed, Garchomp can still outspeed a lot, and 130 base attack means things are going to be dying (there's a reason Garchomp's been one of the most used Pokémon across the board since the 4th Gen!). Crobat, on the other hand sports an underwhelming 90 base attack, meaning it's going to be 2HKOing more often than not if you're not running Choice Band. Plus, Garchomp has a better movepool, such as the elemental fangs, Stone Edge, Iron Tail, Poison Jab, STAB Outrage and Earthquake, and more. As a sweeper, Crobat gets Brave Bird or Acrobatics, Cross Poison, U-Turn, Steel Wing, Zen Headbutt... and that's about it. I mean, you could use a more unconventional Nasty Plot Special set, but even then, you need to use Nasty Plot before your attacks are going to be worth your while.
However, most of Crobat's most commonly used sets are for fast utility. If there's one thing Crobat has, it's speed, which makes Taunt a great option for it. Most sets run some combination of Taunt, Roost, U-Turn, Brave Bird, and/or Defog. One thing to note, though, is that other utility Pokémon, notably Mew, generally outclass it. Mew's still really fast, a lot bulkier, isn't weak to Stealth Rocks, and can learn pretty much everything Crobat can except for Brave Bird (which you wouldn't want to run on Mew even if it could learn it), plus more. Crobat is really just kind of mediocre in this current metagame.
In conclusion, if you need a sweeper, go Garchomp. If you need utility, Crobat is an option, but I would recommend looking at other utility Pokémon, as well.
