What is the sub-floor for? Is it because the concrete is in bad shape or do you want a softer floor for living space?
Is the concrete floor damp at any time during the year? Laying poly over a concrete floor will create moisture between the poly and concrete, then screwing holes down through it will render it useless. A better solution to control moisture would be a penetrating sealer. RadonSeal sells one that controls moisture and radon mitigation.
6 mil poly in contact with concrete will break down over time and become brittle. If you decide to go the "poly" route you need a product like Stego wrap. It is a true vapor barrier. But you can't puncture it with nails or tapcon screws you will be wasting your money.