Trick question; Doesn't matter, provided that the push from behind equals the drag from in front. What matters most in this situation is what position allows you to put out the most power, to accelerate from this break-even, to go faster, if you desire. But even then, from 15 to 20 the drag from the front will be low; (IIRC?), aero drag increases as a square of the speed, and the power needed to overcome that increases as a cube of the speed (it's been a long time, I can't recall the exact reasons for that difference).

There's been a longtime puzzle among sailors, can you sail Dead Down Wind faster than the wind. Simply raising a spinnaker (large sail to use downwind), no. But I think this problem was conquered a few years back using a large prop in some way on a landsailor; It's been known for a long time that sailing a broad reach (4-5 or 7-8 o'clock when wind is from 12) can be so much faster as to get you downwind faster (faster Velocity Made Good or VMG) than DDW, even though a longer path; This is because the sail is generating lift, not simply catching the air like a (round, non-airfoil) parachute, which is what a spinnaker DDW does.

Kudos for your intellectual curiosity, John.