Merritt and I both found Africa much more impacting than South-America but we had more time and we were traveling on two motorcycles, meaning that we got a chance to cross the entire continent and reach fairly remote places, which will be rather difficult for you two-up on a bike.
If cost is really an issue, you might be better off in South-America: you don't need a Carnet, you don't have to pay the visas, and life is generally cheaper or more comfortable than in Africa for those who stay at hotels/pensions and eat in restaurants. If you can carry some camping/cooking gear, and manage to resist most of the time the amazing safaris and scuba-diving spots, Africa stays affordable.
The choice also depends on how much you enjoy talking with people, and how fluent you are in Spanish. Both continents have amazing sceneries but they might be more accessible in South-America: just spend 6 months criss-crossing the Andes from Ecuador down to Tierra del Fuego, and I swear the rest of the world will seem rather flat to you.
On the other hand, in 6 months you have plenty of time to do a beautiful, quite diverse, and still pretty comfortable tour in Africa also, from Cape Town to Lesotho, Mozambique, Malawi, Tanzania, Zanzibar, Kenya, maybe a round-trip to Uganda, and back south through Kenya, Tanzania, Malawi again, then Zambia, Botswana and Namibia.
I don't know if you are experienced enough to ride two-up on dirt roads. In Africa, you can stay on asphalt pretty much all the way south of Nairobi if so you choose, although the desert tracks in Namibia are quite inviting and "easy" (at least for someone who has crossed the rest of the continent). In South-America, you need to ride on gravel roads to reach Patagonia or to cross the Andes in many points. There again, the roads were "easy" for us but some travelers don't have the same experience (http://www.photobiker.com/continents...x.html?lang=en
Overall, South-America (even the more traditional areas of the continent along the Andes) might be a tad easier and more comfortable, while Africa (even the more modern areas of Eastern or Southern Africa) might require a bit more effort. In South-America, it's easier to stay a contemplative tourist; Africa often requires to be a more involved traveller.
Here are some more stories from our web site:
- Recommendations for South-America: http://www.photobiker.com/continents...324/index.html
- Africa by Merritt: http://www.photobiker.com/continents...207/index.html
- Africa by Pierre: http://www.photobiker.com/continents...627/index.html
Either way, you are going to have a beautiful ride. Both continents and their people are amazing.
Pierre & Merritt