After looking at the specs of the M1, IMHO the .21mv version would be right on the cusp of not working well.
It also depends on how much gain your preamp has through the line stage, but unless it's a lot, the 1mv version will definitely work better and be a safer choice since you probably can't return a cartridge once it's been mounted.
It also depends on how much gain your preamp has through the line stage, but unless it's a lot, the 1mv version will definitely work better and be a safer choice since you probably can't return a cartridge once it's been mounted.