US internal flight
 

We are planning a round trip LAX to KOA next January, we are both BA Gold (until April).
Makes sense to use one world, travelling economy what benefit wound we get on Alaskan or AA?

AA would give you access to the Flagship lounge at LAX, Main Cabin extra seating, extra baggage and priority boarding.

So should I book Basic, Main Cabin or Main Cabin Extra We will have 1 cabin bag each and my wife will need assistance from checkin to gate at LAX

The benefits outlined in Post #2 apply to all Economy fare types, including the cheaper Basic Economy. So if you can accept the non-refundability of a Basic Economy fare, then book that. After AA can "see" your oneworld status, you should be able to select Main Cabin Extra seats for both of you at no extra charge.

Basic economy also means very little or no accrual (not sure, check your mileage program rules).
I believe Basic cannot be changed.. you have to cancel and there is a fee to use the credit for another trip.
https://www.aa.com/i18n/travel-info/...ic-economy.jsp

You can only get a future-travel credit when cancelling a Basic Economy ticket if your AAdvantage number is in the PNR. The OP would be placing his BAC number in the booking to get oneworld benefits. (Not sure whether changing the number to an AAdvantage number if one decides to cancel would enable one to get the credit.)

Very interesting point. From the website (bolding is mine). Note the part about canceling via website or app.. implies that canceling via agent does not qualify.

AAdvantage® members may be able to cancel Basic Economy fare tickets and receive travel credit for a fee. To qualify: 

• Your AAdvantage® number must be in your reservation before canceling
• Your trip must be booked in the U.S. directly through American
• Your trip must be on an American marketed and operated flight starting in the 50 U.S. states
• Cancel on aa.com or the American app before the first flight departs

If the fee is more than the ticket price, travel credit will not be issued.