You could go 18/18.
Or you could go 21/15, do an extra point or two of damage per swing, get 5 extra hit points, be a little slower and have a slightly worse shield.
Or you could go 21/18, which is slightly faster than 21/15, but with less hp.
Or you could go 18/21, which is a lot faster than 21/15, does slightly less damage and has less hp.
Or you could get a blunt/pierce weapon and go 15/24, which is WAY faster than 21/15, but does less damage vs lightly armorered targets.
Just figure out what you like to do and play around with numbers. You can change a STF whenever you want to now, making experimenting easy.