There are great choices for each animal and only OK choices that would work well for both. I would question the rationale for “needing” a single bullet for each application. Buffalo/elephant hunters face a similar, but slightly different version of the same question.
My .416 Rigby in a 550 CZ puts 400 gr solids and 400 gr TSX within 1 inch of each other at 100 yards.
When hunting leopard, at least over bait, the distance will be certain and it’s common to sight in the rifle precisely for that distance. Within DG distances (100 yards max), there’s a good chance two vastly different bullets will be close enough that there no real difference.
If so, use a 400 gr TSX (or similar) for Buffalo and use a 300 or 350 gr much softer bullet for the cat. However, with the cat, absolute precision is important. Most of the time, something fast in the .300-.338 range would be a better choice.
If I had to chose one, I’d go with a 350gr A-Frame, if you can find them.