I am no fundi on Voyager (and even less so on SQ) but...

You can get Silver status based on 3 sectors of >4000 miles - Gold for 6 sectors and platinum for 12. If I read your post correctly you will actually be doing 24 of these sectors (12 returns CPT-LON). That would make Voyager an ideal programme for you. Even if you travel on fares that attract reduced mileage, the other benefits would kick in very quickly and you would get additional tier miles to compensate - at least partially.

I'd be less worried about the miles earned than the lounge, *G status and other perks if I were travelling in Y. And from what we've heard around here, SAA really looks after their Plats. If you're planning on maintaining this pattern for any length of time I'd seriously consider Voyager.

Plus I always think it's a good principle - though certainly not an immutable one - to join the FFP of the carrier you will fly most often.