I do not know the Sydney route but have used BA + VS on the Johannesburg and Cape Town routes for over ten years. Last trip to JNB was with BA in August.
If seat width is important than that gives you the answer. For a choice, I would say that VS just takes it ahead of BA but only slightly. The advantage is to do with the new cabin décor, wider seat and general 'refreshing' of the PE cabin.
Service and food are almost identical with service depending (as ever) on the particular crew and circumstances of the sector. I will happily travel with both of these carriers in PE/WT+ again.