Which one is better for economy long haul London <> Sydney? I think I like Singapore Airlines better because the flight attendants are godly-friendly and food and service is great (also with free cocktails during flight), also the stopover at Changi Airport is awesome. Airport has cinema, good food and you can get the $20 free voucher to spend there.
But at the same time, Singapore Airlines for this flight is running Boeing 777 and Etihad has Airbus A380, which I think is a better plane and it's really quiet. And Etihad is slightly cheaper.
And unrelated, but if anyone is travelling as a couple; is it really a good plan to pick seats near the back that are 3 abreast and you book the window and the aisle? Then most likely no one will pick to sit in the middle and even if they do, you can ask if they want to switch to the window or aisle so you still get to sit with your partner. Never tried this plan but I've heard success from others.
No question, the A380 is better than any 777. I'm doing a similar route myself next week, Melb to Europe on a Qatar A380. A point to note is transiting through Sg breaks the trip into 2 similar chunks, rather than the 15+7 hours on Etihad.
That being said Abu Dhabi is TERRIBLE! Changi any day!