I'm the same height, like 5'10.5", i ride the 17" but i also felt like maybe I'm in-between sizes. I have a lot of post showing, but i suspect the standover'd've been too high with the 19". The toptube difference isn't very big, if i recall (redline has full specs/geo on their website), and you can always mess around with stems. I have a 17" with like a 90mm stem, b/c i don't like to be too stretched on the bike.
The best thing you could do is find locals who rock the monocog and borrow theirs, to see what fits. I have a freind with a flight in 17", and while the geo is different, both frames fit me quite similarly. If you can't run that, i'd sday get the 17", try it out, and get a 19" frame to swap the parts too if the 17" doesn't work out.