Since Anglo owns about 65% of Kumba, you can get some Kumba through Anglo. A play on Iron as opposed to a more diversified resources maybe. Both look fairly cheap at the moment IMHO. But I think given the choice I would go with KIO.
I guess it all hinges on your views regarding the platinum industry, which is weighing Anglo down. If you believe that platinum is going to turn, then the investment case for AGL is really strong, because you get the best of both worlds - exposure to iron ore profits - and the bottom of the plat cycle. If, like me, however, you believe that platinum is still in the doldrums, then I would stick with KIO. Personally, long term, I wouldn't take either of these right now. Resource stocks are always volatile and tricky to invest in, so my preference with them is to bottom pick - i.e. wait for the real selloff. I also have a personal preference for Exxaro over KIO. Exxaro is a shareholder in KIO - so you get the iron ore exposure, and it gives you coal exposure as well - which is still a hot commodity