I'm not a fighter play, but here's my take anyway:
No restrictor: great for analog games, a little long for shmups, has a "soft" end of movement.
Round restrictor: okay for analog games, great for shmups, has solid end of movement
Square restrictor: okay for analog games, can feel corners, has solid end
Diagonal restrictor (aka square restrictor rotated 45): great for 4-way (can feel cardinals), has solid end
My guess, you'd like to use one of the restrictors, depending on if you like to feel diagonals or not.
Also, mame has copied the u360 9x9 analog to digital conversion method starting with 0.117u2. So even with FEs that don't switch modes, if you set the u360 in analog mode, mame will do the conversion automatically.