My logic has been flawed all through this tournament to be honest BUT the Argentina have recent history of beating New Zealand (twice). Teams who beat Ireland in a QF, historically always lose the SF and last week will have exhausted New Zealand. IF Argentina don't fall behind early then they can frustrate the All Blacks and in an arm wrestle it's 50/50. But I don't think that'll happen and New Zealand will probably be too strong as they can score out of nothing.
As for the other, I just don't see a single way that England can beat South Africa besides some freak events happening. They kick better, they attack better, they defend better, they hit harder etc.