Ugh, the worst is trying to decide on a Nature for my in-game Magearna. I recently looked up options for Tapu Koko and Lunala they were pretty obvious, but Magearna... There are just too many options! I'm trying to think of some simple speed tier comparisons to weigh the options between Shift Gear and Trick Room. This'll be really brief, so please do add some relevant threats that I miss.
Modest Magearna can reach a maximum of 229 speed, being doubled to 458 with Shift Gear, so this is purely in the realm of Choice Scarf users now. At max speed, a base 90 Scarf goes to 459, and Pokémon who can reach that speed that can pose any threat to Magearna include Garchomp, Genesect, Jirachi, Landorus-T, and... er, Darmanitan (watch out everyone, Darmanitan's making its comeback as the premier Magearna counter, for sure). Magearna's typing and its defenses really save it, as there are a lot of Dragon-type choice scarf users (like Latios) that don't really have anything outside of Hidden Power.
Quiet Magearna drops to 121 speed, and I can't think of anything that runs a speed lower than that aside from, say, Quagsire or Ferrothorn (whose matchups against Magearna don't really change much depending on whether or not they get to go first; I guess Quagsire can hit her with Earthquake before it gets vaporized by Fleur Cannon, but that's about it).
...Maybe Torkoal's making a comeback (as if it was ever relevant) with Drought?
I think Trick Room lets you bypass more relevant threats, so it comes down to the fact that Trick Room is on a timer and you don't have the option to turn it off for the other Pokémon on your team, but those are just general "Trick Room vs. speed boosts" considerations that everyone should know by this point.