Better to buy as a codeshare?
 

I'm going to buy a business class round trip flight that costs ~$3000 and the distance is about 20,000 miles. The flight itself is a operated 1/2 by AA and the other 1/2 by BA. I have the option of buying the flight as AA marketed (with all AA flight numbers) or BA marketed (with BA flight numbers).

As an AA Gold it seems to make much more sense to me to buy the flights as BA marketed:

AA Marketed:
Award miles: 3000x7 = 21,000
EQM: 20,000x2 = 40,000
EQD: 3,000

BA Marketed:
Award miles: 125% x 20000 = 25,000
EQM: 20,000x2 = 40,000
EQD: 25% x 20000 = 5,000

Am I correct here?

You are correct. Well almost.

I think award miles should be 165% not 125% as you get 40% bonus from your gold status. This means 33,000 award miles for the BA booking.

Only consideration would be if you're AA flights are on a 77W and you wanted to upgrade, as I don't believe you can do so on a BA flight number, but otherwise yes seems that you're better off in booking the BA flight.

I'm not sure why you think you won't earn AA RDMs, EQMs, and EQDs flying BA metal with BA flight numbers LHR-India. The Partner Earning Chart for BA states:https://www.aa.com/i18n/travel-info/...sh-airways.jsp

Every fare basis in the chart earns something.

I guess that the person was inferring that , since India is not in that list of eligible routes, that there would be no mileage accrued travelling on BA on that route

Well, you do earn on LHR-India routes - just in case that really was the concern here. I think they consider India as part of Asia.

I have 4-6 LHR-MAA (or reverse) segments on BA a year - usually I fare - always get points.

If LHR-India does not accrue miles, I will be a lowly Gold instead being EXP :)