With status if flying in Y, I would always choose DL. You get free Economy Comfort and PTVs as well as 100% MQMs and a chance at an op-up if the stars align.

Without status, it is a pretty good toss up. Many believe the service and atmosphere on VA is superior.

If you fly VA, your elite benefits are not going to be recognized like they are at DL and an Economy Comfort seat is going to better than a standard VA coach seat. I would definitely go with DL for any passenger with status and access to free EC. If it is just a trip in Y and you had no status, I think I would probably try VA since most international carriers offer a superior coach and business product to US based airlines. Plus I wouldn't care so much about the miles. Currently VA flights do not earn MQMs and the lower fares only get 25% and 50% RDMs....http://skymilesoffers.delta.com/airl...php#Vaustralia

So, you may want to be careful if considering VA. I'm sure that in time it will change since they just announced the expansion of the codeshare agreement. I think if you buy the DL designated/VA operated flight you would get MQMs and RDMs like normal since it will post in your account as the DL flight number but I could be wrong.