For the outbound, I believe the JNB-SIN on SQ is on a two-class a/c, whereas there is a JNB-BKK on TG in a three-class a/c (346 i think). Obviously SA has a lot more options for the outbound, but if you're traveling in first, I think the TG flight JNB-BKK is the only *A flight continuing east that has an actual first class.

Not sure your class of travel, but food for thought. I also didn't look any of these up, it's just what I recall when toying with an RTW a few months ago that was eastbound and stopped in CPT. So while considering C service on SQ versus C service on SA, realize the SQ might be on an older 2-class a/c without the new C. I would do a dummy booking on SQ for one-way JNB-SIN to figure it out.