12 Gauge or 20 Gauge for Deer/Upland?

Im going to buy an 870 Express Combo but am undecided on what guage to get it in. Im leaning toward the 20 as it has less recoil and slightly better ballistics in terms of shooting slugs, but in terms of shooting shot does the 12 carry much more shot to make a real difference while shooting pheasant or clays?