Both are certainly adventures. You left out Argentina which also has some excellent buffalo hunting. My advice - DO BOTH!
If you have not hunted Africa previously, I would look into going there since there is SO MUCH to do besides buffalo. In Oz you're pretty much limited to buffalo and maybe some pigs. In Africa, you can do all sorts of other animals in addition to the buffalo.
As far a challenge goes, my experience is that the Oz buffalo pretty much stand around and you can pick and choose. A Cape Buffalo on the other hand will lead you on a merry chase and you will probably not be successful on day one, and probably not on day 2,3,4 etc. But, eventually you will be successful and you will cherish the trophy all the more for the experience. Be careful who you book with in Africa. South Africa and Namibia probably have the most outfitters and are the best developed as far as the hunting industry goes but most hunting there is on "game ranches". Zimbabwe and Mozambique are much more "free range" type hunting. Tanzania is the priciest but also has some excellent hunting.
Best of luck in your endeavor. I say again, do both as both hunts I have taken have been very memorable.