One is an FX fwd position, the other one is FX spot... That's the fundamental difference. The technical difference is between the need to roll the spot position (a spreadbetting shop does it on your behalf) vs no rolling for the FX fwd. Every roll transaction gives you your positive or negative carry, but incurs a cost (for instance, IG does the roll by trading out of an existing position at mid, then buying/selling the tom-next fwd at bid/ask + fixed fee of 0.55bp x price). You should check with your S/B shop for the exact details...