If you are looking for upgrades on transatlantic, KL may be your best choice: it will only require 20000 miles each way and you can upgrade from any fare. If you are Gold or Plat on KL, you will earn not far from that for each return flight, so that would be definitely worth it (as long as you fly KL metal across the pond, as upgrades on NW and CO are much more difficult to obtain).

If you are flying BA on discount economy, you would probably be better off joining QF than BA as you will only get a fraction of the mileage with BA with the lowest fares, against full mileage with QF. If flying full-fare or near full-fare eco, it would probably be better to join the BA exec club.

In terms of service, you'd get a better service on BA than on KL or CO in whichever class you fly.