Well, the Jeffery fires a slightly wider and heavier bullet slightly faster than the Gibbs. Because it uses a smaller case, pressure is higher. Supposedly this matters in Africa, but after decades of reading I've come across exactly zero reliable reports of either one failing to extract because of pressure. Both cartridges are notoriously difficult to make feed in a rifle. Both have sterling (albeit quite limited) reputations against the largest game. Both recoil ferociously. (And yes, in theory the Jeffery would recoil slightly less, if you fired the same bullet at the same speed. I'm not sure you can even find bullets of the same weight for both cartridges, and even if you could, the few ft/lbs. of recoil would be utterly lost in the big picture of 100+ ft/lbs.! The reality is that the Jeffery drives a bigger bullet faster, so recoils more. Of course, the "big picture" note still applies.)
Essentially, they are equally useless for anything but situations which few of us will ever have the good fortune to encounter. So it becomes a matter of pure emotion. My emotion said "Buy the big one!". I simply prefer the charisma of that giant .505 case, and as long as we're choosing the CZ with the giant action, we might as well choose a cartridge that fills it!
