--freq=850 is the setting the clock speed for all the Gridseeds.
--gc3355-freq=/dev/ttyACM0:900 is setting the clock speed for a specific Gridseed. If you have multiple, you can set them like so: =/dev/ttyACM0:900,/dev/ttyACM1:850,/dev/ttyACM2:950

You can use either. I don't know what'll happen if you use both though, presumably the --gc3355-freq is more specific and will override whatever is set on --freq. If this is the case, you can set a default clock speed for all the Gridseeds and override a few that need to be set differently.