VA in a heartbeat. I haven't flown DL specifically on LAX-SYD but have flown VA down to OZ (BNE) in J and have flown DL in D1 on several other routes and while D1 is pretty decent, VA is superior in just about every way. The seats were better (more comfortable and the window seats faced towards the window rather than away from it). Entertainment was just as good. The in-flight bar was nice to walk-up to and sit at.

If price is the same/similar and neither's schedule is more or less desirable, I would highly recommend VA. I did a trip two years ago to OZ (Australia and Japan). Flew five airlines on the trip: VA, CI (China Airlines), NZ, SQ, and DL. All in J. I enjoyed VA about as much as I enjoyed SQ while on the DL flight, the best plus was it was on a 747 on the upper deck but other than that DL's D1 product was simply not as enjoyable as the other products.

If you're in J, your status won't have much impact or make much of a difference. VA should still recognize you as a partner elite but again, little impact here with being in J anyway. You may want to evaluate which one will give you better MQDs/RDMs depending on who you book it through if that matters to you (Booked through VA, flight would earn RDMs based on distance while booked through DL as DL flight numbers would earn based on the spend).