I would also choose OZ.
Both the A380 and A350 offer an excellent experience from the point of view of cabin air and pressurisation. I prefer OZ business class seats, SQ are too hard, too wide and on the A350 they are still the ones with minimal recline that you have to stand up and flip over if you want a bed. At one time on SQ the crew came and made your bed, now there's a leaflet telling you how to do it yourself so far has SQ cabin service fallen.
Food on both is comprable, perhaps more interesting on OZ, SQ tends to play it safe.
Crew on SQ perform to script and don't deviate, crew on OZ are far warmer and more willing.
OZ are on the way up, SQ are on the way down and IMO (as a top tier frequent flyer there) are trading on past glory, there was a time when they were the best, they certainly aren't now and I wouldn't put them in the top 25%.