In order to identify the value for you here, it's worth taking into account your objective. If you consider the welcome bonus worth the annual fee then yes, get it for a year, use the points and cancel it.

But moving forward, do you value getting 1:1 amex points and transferring them to your desired location over 1:1.5 avios? if the former, you have your answer, if the latter, then you have it too!

I currently own both (and the US plat which beats both by a mile). Putting aside the US plat; to me the value I get from the BA card, is 'better' as I get more points per £ (and I primarily use BA over other airlines), the annual fee is less and as you say, the 2-4-1 if you're hitting the threshold. If I did not have the US plat, I would get significant value from the centurion lounge and would maintain this too, however this is only relevant if you visit locations with them.