I had the same choice for a trip next month, albeit in J, and went for QR.
However, for Y travel I'd probably be tempted to go for QF operated by EK if all legs are on the A380 - Y on there is spacious and rather nice from what I've seen of it.
Can't answer about seat selection I'm afraid as have no experience there - the Qatar forum suggests that the mini cabin is reserved for their own elites, but a phone call to their CS or asking at check-in may wield a positive result