First, I must qualify anything I say.

I have no african big cat experience and my only big cat was a cougar loping at me which I shot cleanly with a lever action.One shot-lights out. The professional hunters using dogs in Colorado use handguns/small caliber rifles to good effect.

The african leopard is a totally different beast, a true terror.
Given my own choices I would probably choose a rifle for precision shooting and my usual big game deep woods followup gun- a beretta semiauto with a 5 shot extension shooting benneke slugs.I personally don't feel that buckshot would get the job done reliably with the penetration needed.Apparently Mr. Africa-Boddington( no detraction meant only envy!) states he prefers projectiles other than buckshot for leopard followup.

Lacking the ability to take the Beretta SA to Africa, I may lean to the Drilling with slugs.And no, I don't think there will be time for 3 shots. With the Beretta, maybe 2 shots,More like 1 aimed shot and maybe a "prayer" shot with the drilling if I'm real lucky.

Excellent discussion by all here!!!