I have a 19x18 on my Honda CBR 929 and I love it. The gix may get one someday...
That's my understanding, too, that the lever is harder to pull on the 19x20 and has less travel, whilst the 19x18 has a lighter pull with more travel.
Hey, get the best of both worlds...they make one that's adjustable, so you can have both and switch between them as you see fit!