Generally LHR gets the best equipment that SA has available (due to having to compete with BA, particularly for J business). JFK and ATL tend to have worse-condition planes than LHR or even ZRH or FRA, and captive routes like HKG get the really bad planes. SA has started a long-overdue fleet refurbishment, and several planes are nicely refitted; you'll be happy if you happen to get one of those (though the food is still sub-par). I believe more than half of the planes are OK now, so you have a better than 50-50 chance in general, and probably 80% out of LHR. But every BA plane is superior to the best SA plane, hence the preference to use BA. The one advantage you have flying SA is access to the real SA F lounge in JNB (though I've heard they don't admit award-ticketed passengers, but you should try, it really is a beautiful lounge; also eat in the lounge, the food is much better than on board). As for mechanical safety, I think SA is a safe airline and well run, with the 747 fleet in good mechanical condition (it is the interiors that are run down).