Hi Fiona,
From personal experience I can give my comment on Virgin Atlantic Premium Economy as I have never flown Qantas Premium Economy I have however had discussions with friends that have flown both:
Virgin Atlantic are the pioneers of Premium Economy, first introducing it in 1992, so as you can expect they lead the way and beat Qantas in most areas, for a start their seat is wider 21 Inch compared with the 19.5 inch of Qantas.
General concensus is that Virgin Atlantic's in-flight entertainment sytem is far superior to that of Qantas, then there is the price, cheaper than Qantas.
In my experience the service I have receieved over the years on Virgin Atlantic has been far superior so I don't expect that to change when comparing Premium Economy between both carriers.
Finally the lounge, I find lounges at least in Asia, Oceania and Europe much of a muchness, unless you are lucky enough to fly First Class.
For all these reasons I highly recomend Virgin Atlantic over Qantas, the only reason I would recomend Qantas Premium Economy is if you were trying to gain a higher status in Qantas/Oneworld