Lots of things to consider really and one could faff along forever. BA Gold other than being emerald offers you very little as far as upgrades are concerned. I was one for many years. I cannot do the calculations, but if a lot of your travel is on AY coded, then definitely AY provides better value with upgrade vouchers and regular SMS offers for miles upgrades. I did the swap on similar grounds to AY, made Platinum in 5 months, but I am travellin the LH in J. The only other thing to really consider is that Finnair in recent months has become quite an expensive airline to fly and now my flights to Asia have pretty much all switched to Qatar.

Anyhow, when choosing between the two, I think you get the best value by choosing the program on which your flights will be ticketed with. If there is a lot of BA, i would go with them and if a lot of AY, then with AY.