Hi,
I've just gone through this decision myself, but for a SYD-MUC-LHR-SYD holiday in December/January (Lady Lenny and I are fans of the cold!!)
I was quite tempted to try EY for the first time, but in the end I chose to stick with SQ for a number of reasons, but for me there were 2 main factors: I know the Y product on SQ quite well, and I know that whilst it's Y, I do fit in the seats (I'm 6'3' and used to swim/play rugby) there is reasonable (for Y) legroom, and I am happy with the service that I get on board. EY seemed a bit too hit and miss in terms of Y service and product.
Secondly the transit experience in Chang is efficient - SQ from Australia and to Europe arrives and departs in the same terminal (if we're lucky, sometimes from the same group of gates too) Changi Terminal 3 is well layed out and handles the crowds at the usual Oz-Europe transfer times quite well, as long as the transfer is under 3hrs then I don't have a real problem with the KrisFlyer Gold lounge for a quick drink and a snack.
From what I have read on AFF and in other places 1hr 45min would be pushing to get through the terminal and then to your gate, let alone have time to properly enjoy an F lounge. Especially if your inbound or outbound flight is on a remote stand. For us that was pretty much a deal breaker as I try to avoid as much stress as possible when travelling, and a potential crush and rush at AUH with a very tired Lady Lenny (who struggles to sleep on planes) without even any real chance to sample the lounge.
Mind you, this is all based on my reading of other peoples opinion, and our own specific needs. I've been transiting via Changi since I was a boy whenever we did a trip to Scotland to visit family and have done a few work trips to Malaysia on SQ so I guess SQ/Changi are my default settings. Others may have different views or experiences
Edit: Forgot to mention earlier - pretty much no chance for upgrades on SQ, they are almost impossible for non KrisFlyer members and rare enough for KrisFlyer members!!
Your lounge options will be broadly similar at either end: SilverKris (which is quite nice) or Etihad Business/First Lounge (never been, but looks similarly nice) in Sydney. In Europe you will get SilverKris access where they have one, but if there isn't one then you won't get access to their partner lounge (at least not using your VA WP status) From what I can see EY may get you access to some of their partner lounges, but I'm not sure how reliable that is. You will of course have access to an EY branded lounge (but there aren't that many in Europe - just Dublin, Heathrow and Paris)
In Changi you will get access to KrisFlyer Gold (basic lounge, drinks, snacks, some hot food/sandwiches. Some people hate it, others tolerate/don't mind it. No showers and the toilets are outside the lounge and just down the hall - but clean and quiet) I find it better than sitting in the terminal, but YMMV.
In Abu Dhabi you will technically have access to the Etihad First Class Lounge - but with that connection time, probably no chance to actually use it