I'm reluctant to call us favourites because even if we got Amla and de Villiers cheaply we still have to score runs against Steyn, Morkel, Abbott and Tahir.
By "less than 50" I mean a random number of runs between 0 and 49. So not necessarily 49 (28). If they both hit 48 (8), to take an extreme example, it would be different.
But if we get them both for 25, say, I reckon we're ahead by an appreciable margin, given that Faf, Rossouw, Miller and Duminy are all mediocre bats and de Kock, although excellent, appears to be in poor form, and that their tail starts early and is probably the weakest of the four teams remaining.