Hello Adam12,
So as not to lead anyone to believe that I have ever hunted either ogf those sometimes naughty kitties, I have not.
However, I've seen both, in The Kruger Park, and while hunting other species, outside the park boundary.
There are so many leopard in the Lowveld of Limpopo District, S.A. that, daylight sightings are not all that rare.
The point? (here's an idea Velo, when telling a story........have a point).
Even though I've taken my share of non-dangerous game with various calibers and styles of "iron" sights in The USA, as well as in Africa, plus one dangerous animal via an iron sighted double rifle......nonetheless, big cats inspire me with awe, more than the rest of the dangerous ones in Africa (grizzlies in thick vegetation or in darkness do too).
Therefore personally, I would not risk wounding one by relying on iron sights, especially in the twilight that they, (almost always leopard), are often, if not usually shot.
Speaking with my favorite PH, as well as any other PH that will hold still long enough to put up with my child-like questions about such things, they all seem to favor Austro-German scopes #1 and Leupold #2.
Both species are typically taken at close range (besides, an animal is not really "dangerous" game, unless you are close enough for him to bash you around after only a short charge.)
Some PH's will not agree to guiding a client for leopard with iron sights only.
Again, I have not hunted either if I ever do ($$), I would use either caliber you mentioned (I am a rifle enthusiast and then a hunter) but with a Zeiss fixed 4x (non-variable power).
I don't believe those are in production at the moment but, the an internet search often turns them up.
Not cheap but, better than having your optics fail half way around the world.
If you simply must have a variable power scope, I recommend the Zeiss Conquest 3 to 9x (also recently out of production but still available on the internet).
I convinced one of my friends to put a Leupold (wanted to save money) fixed 4x scope on his new .35 Whelen (his go-to rifle for Africa and some Alaskan species as well), and he has had nothing but excellent results with same.
Jaco Strauss is a PH and recommends a 4x (great minds think alike).
And, I agree with both Jaco and Grady that, both scope and iron sights on the same rifle are a better than good idea.
Jaco recommends also bringing a spare scope, I have done just that (fortunately did not need it) and it is little extra to bring in your rifle case.
My parting shot (sorry) is that for hunting other than rodents under about 300 yds/meters, I have had the most excellent results with Hornady round nose bullets.
For what you plan, also the Woodleigh RN bullets seem like they would be about perfect.
Mine was a lot of talk from someone who has never hunted either species you are after.
Your best bet is to follow your PH's recommendations in pretty much all things but the above is my dos centavos worth anyway.
Good luck to you,
Velo Dog.