Or, you could take the question at face value and actually apply your mind to it.
Points to consider;
The standout feature of the Boks, and something most seem to agree on, is the culture and how close the team is. Exactly the same type of culture and brotherhood that the 2014 ABs are described as having. I'd say the culture is a controllable. Something no team has an excuse not to have.
Both Rassie and Rennie might struggle to achieve that were they coaching the other's team because a lot of that culture is rooted in nationality. All things being equal that should see both teams being slightly worse off with Rassie probably being able to instil slightly more gees into the ABs than Rennie could at the Boks.
It would probably come down to whether Rennie could get more out of the Bok backs than Rassie could get out of the AB forwards and also how far away each of those teams currently are from their ceiling in those areas.
I'd say the Bok backs are considerably further from their ceiling than the AB forwards are from theirs. I'd say the gap between the AB forwards ceiling and that of the Boks is smaller than gap between the Bok back's ceiling and that of the AB's. The ABs just don't have the forwards that we do while our backs are on par or better than theirs.
The next thing I'd think of is how each coach's style matches what the squad is capable of. Rennie won't be able to run the Bok backs as much because he'd be draining energy from the Bok forwards and they are, as it stands, its major weapon.
Could Rassie push the AB forwards harder without breaking the speed of ball they currently provide for the ABs? I think so. I think he could go for a 6-2 and blast his pack for 40 minutes and then bring on a new pack.
It's actually a damn interesting question, Moz.
Considering personnel, there are a good few players Rassie would instantly drop. I think Dmac would be first on that list. The little glory boy that is too afraid to make big hits or to jump for highballs(I'm massively irritated by him as a player). We know for sure that Rassie will not stand for that. As Eben said, "If you want play for the Boks you must be willing to put your body on the line." Rassie would not leave players like Du Plessis at home and you would probably see Caleb Clarke become a regular feature too.
If recent reporting is anything to go by, some of the AB failures on this tour have been down to lack of prep. Working on their golf handicaps instead of catching high balls. Rassie won't stand for that. I have a feeling that if you struggled to catch high balls and even dreamed about going golfing on Wednesday, Rassie would drop you. And rightly so. I get that the pack needs a break but if you're some outside back that didn't catch a single ball kicked on you on Saturday, you should be putting in twenty hours of high ball reps every week until you sort your shit out. It's actually nuts to think that a team so known for its "skills" hasn't been able to catch a high ball for three years and nobody has thought to fix that. Inexcusable. Rassie will sort that out immediately. Any competent coach would. And if you do all those reps and still can't find parity under the high ball, then you're not a Test player so bye bye Felicia.
...interesting note. Our cricket coach would hit his pitching wedge for us at cricket catching practice...in primary school. Perhaps that's the "No DNA just RSA" I keep hearing about haha.
There are a whole bunch of things to consider with a question like that. Too many to put into a post.
All in all, I think Rassie would clean up the AB set piece and probably try to use more of a pod system on attack, and focus less on the backs. And that would clash with AB DNA. Like Razor, he might lose the dressing room, and the public.
Rennie would have a better time if it because, just like every Bok coach before him, he would have a very solid platform to work from and likely have an easier time delivering a well balanced mix of forward and back play.
So my answer would be, Springboks.