In your situation I would likely bank to Delta. Note: assuming low economy fare (not expensive coach or business/first class). Calculations are different otherwise (but I don't think it works out a lot different).

https://www.virginamerica.com/cms/el...rgin-australia

20% of LAX-MEL-LAX, then 1.3x: 4118 Alaska miles. The Delta flights will not credit.

http://www.delta.com/content/www/en_...australia.html

50% of TPA-LAX-MEL-LAX-TPA: 10,079

I'm not a big fan of stranding a small number of miles in a program where they aren't usable, and I don't value a DL mile as 1/2 AS, especially at a small amount in a station (TPA) where AS has one flight a day, and where/when AA has worse redemption options than DL on DL, as an AS partner. If you were thinking of piling up a LOT of miles in a program (AS or DL) this changes things a bit, but as one-offs/intermittent use, larger pile = better. If nothing else 10K DL miles is $100 off a future flight in conjunction with the DL AMEX.

http://www.delta.com/content/www/en_...ith-miles.html
http://onemileatatime.boardingarea.c...ay-with-miles/

I know CX F/JL F/QR F/EY F are sexy and all that but you should check this thread out:

http://www.flyertalk.com/forum/ameri...iscussion.html

DL does very bad things with their award chart, with no notice, admittedly, but AA is terrible in other ways. Paying for positioning flights because AA doesn't offer any low awards out of TPA changes the value (and makes travel a bit trickier).

Pretty much.