Africans : more fish = less aggression, lots of movement and colour of all kinds
Americans: less fish = less aggression, a few large fish that can or can't have good colour.
I've kept both and the only downside to africans is more fish means that you have to spend more money stocking the tank. If money wasn't an option I would go africans every time. American cichlids are interesting and they have cool personalities but you always end up being flamed for having too small a tank or incompatible species. You will see guys with huge 200 gallon tanks with 1-3 fish in there. IF you want to constantly worry about your fish fighting or killing anything new that you had (I had that problem a lot) then I would go with the Americans. Like I said I have kept both, and cichlids in general for over 10 years and I prefere africans for the reasons stated.
Plus the south/central American cichlid keepers tend to be a lot less forgiving of mistakes