It really comes down to schedule and if you can make it work. Alaska is not going to meet UA on frequency, but you might bring in a lot more miles if you can live with Alaska's limited schedule. Also, Alaska's award table, particularly for international travel, can be tens of thousands of miles less. I'm flying home next week in Qantas first class for 70K. An equivalent award on UA would be 90K, and on AA 110K. Sort out where you want to use your miles and that may impact your decision, too.

Tom in transit at SYD