I have been present at the kill or taken lions myself on 23 occasions over the last twelve years. Most have been taken with .357 mags with either 125 grain CorBons or 158 grain Corbons. Only one required a second shot. A couple were taken with .45 Colts with factory loads. One was taken with a .30-30. One of our cowboys tried a .32 Magnum and the small female took two close range shots to knock it out of the tree and still required a finisher from a .357 to protect the dogs.
I have not seen as many bears taken (and have never killed one myself). I do know that one treed bear required five shots from a .38 Special to get it out of the tree and a cylinder full of .22 Magnum Rimfires to put a finale to the rodeo. 240 and 260 grain .44 Magnums work well, as do .357s with 180 grain bullets. The only bear that I have seen killed with a rifle was with a .45 Colt in a Model 94 Winchester--it was dead when it hit the ground.
If your hunt is a rare oportunity or a "once in a lifetime" hunt, I would go with a .44 Magnum for bear. For lions, a .357 is sufficient. This presumes that you are familiar with your pistol, use suitable bullets, and that you can place your shots where they need to go.
