Which way do I use less margin: a leap diagonal call spread or substituting a single stock future for the Leap and writing the same OTM call?
You can't buy diagonal spreads on margin. Long options have no loan value so you cannot borrow against them. They must be paid for in full. Therefore at best, the long leg is reduced by the premium taken in from the short leg. This assumes that the long leg has the same strike or is ITM. If it is OTM, then the short leg is not covered for margin purposes. I don't know anything about SSF's.