id say the 18.6" would fit u better for your size, but in the end it comes down to a matter of preference.
if u are riding an 18.8" now, either frame wouldnt be too far off from what youre used to already.
if u are comfortable on the 18.8"- ask yourself this, "do i need it to be a hair longer? or a hair shorter"?...like if u think u need a bit more leg room-go with the 19". but if u think u need a bit less room-go with the 18.6".
we can all sit here and think we know whats best for u, but like i said, it comes down to u.
ask yourself those 2 questions while youre out riding the frame your on now, and take it from there.
peace.