The higher setting is the starting point, because that’s what Ortofon specify. However, no harm whatever will come from trying the alternative setting, and you might prefer the sound.
Matching MC cartridges is as much art as science, and each user has to find their own way. Ortofon are still quite keen on using transformers, whereas other swear by amplified stages.
The gain will be set high because the output is low. That’s the purpose of it. Don’t assume however, that you’ll get the same volume as from say CD, because even a high gain phono stage may only output 0.5 to 1 volt, whereas CD players and streamers mostly exceed the original 2 volt spec these days. If you prefer the second-highest gain setting on your phono stage, and simply turn up your Michi more, that’s fine too.