Although the pressing movement is the same, the higher the angle the more difficult the lift b/c of gravity. The more angled the sled the greater the component of force is pushing back against you.
All I can say is the old school vertical leg presses (which I think were the best of all the leg press), were a lot more effective then the angled/horizontal leg presses they have today. You cannot "relax" when in a vertical leg press, whereas with a horizontal leg press you can.