I'm not trying to bash those two players as I rate them both massively. I also wouldn't question their bravery. But, in a Autumn/Winter test match in England against the Boks, size matters. Look at the teams that have been successful in the Champions Cup, The Premiership and The 6 Nations - it's overwhelmingly dominated by the teams with larger, dominant packs.
If those two play, these are the likely match-ups in the pack alongside how the England player stacks up:
Hepburn 6'1, 17st v Kitsoff 6'0, 19st - similar height, 2 stone lighter
Hartley 6'1, 17.5st v Marx 6'2, 18st - similar height, 0.5 stone lighter
Williams 6'3, 20.5st v Louw 6'1, 20st - 2 inches taller, 0.5 stone heavier
Itoje 6'5, 18st v Etzebeth 6'8, 19.5st - 3 inches shorter, 1.5 stone lighter
Kruis 6'6, 18st v |*Snyman 6'9, 18st - 3 inches shorter, similar weight
Underhill 6'1, 16st v PSDT, 6'7, 19st - 6 inches shorter, 3 stone lighter
Curry 6'1, 16st v Kolisi 6'2, 16.5st - similar height, 0.5 stone lighter
Morgan 6'3, 18st v Vermuelen 6'4, 18.5st - similar height, 0.5 stone lighter
*(or if they pick Mostert 6'6, 18st)
I know it's not all about the stats but a few inches and a few stone here and there can make all the difference, according to my ex-girlfriend anyway...