I've heard comments from Thomas users that the Poly barrels use quite a bit more air, especially in the shorter lengths. I assume this is because the lesser resistance in the Poly does not build the back pressure (as quickly) to close the valve the way a conventionally rifled barrel would. Makes sense to me.
So, if you can't decide which length -- "between 17.5 and 20 inches" -- I would pick longer within that range rather than shorter to preserve velocity and to conserve on air.
You can always cut it shorter later, but you can't add barrel if it's too short.
Just a thought, FWIW...