I'm exactly your size, and use a 256 frame (hardtail) 19"
But it depends on your inseam too.
If you can measure your current distance between the top of the saddle and the bottom bracket of your current bike.
Then make a little drawing from the geometry file ( I use sketchup to do this ).
You will quickly see how high will be your saddle compared to the headset, and the distance between your seat and the bar.
The saddle may end to high over the bar on the 17.5 (you can put spacers below the stem, but generally not more than 2-3cm, or these ugly riser stems
).
Or you may have the bar too far from the saddle on the 19 (bringing the saddle forward, and a shorter stem may help a better fit).
Measure the stack and reach of your current bike and compare it to the 036 may help to comfort you in one or the other.