I haven't been to either one (yet), and it's probably going to be a choice for a once in a lifetime hunt for me. So.....I've researched both extensively--not the same as being there, but a whole lot cheaper. Here's my take.
Australia is much more of a hunt. You can spot and stalk in Africa, with much less chance of success,and you will need a lot more time; or you can set up over a water hole and it seems be pretty much guaranteed several shot opportunities. I'm sure it's not as easy as it sounds, but to me it's basically like hunting over a bait pile--the animals got to have water, water supply is limited, so there you go. If you sit quietly long enough, they will come.
I dreamed of Africa for years--mainly because of the many different animals--but after talking with friends who have "been there, done that", I don't think I'll ever go. It really broke my heart--I have lusted over the thoughts of having gemsbok and kudu and wildebeast--even warthog--hides and mounts for years. One friend who has been there, and may go back, described killing one particularly sought after trophy to be like "shooting a cow" (and he did it with a recurve). He said if he does go back, he will never shoot another one of those.
It's just my opinion, so take it for what it's worth, but the choice for me was easy. I have my bow on order, and plan to do my hunt in Australia--Good Lord willing, in '06.
Chad