Why do you say that about Steam? I bought COD on Steam specifically to try out Team Fusion's mod, which has been extremely popular, and completely changed the game (again, see Simhq for the reviews of the latest mod from them). It works with the Steam release the same as the first release, although with both it is a bit of a chore to install Team Fusion (4 seperate downloads that need to be installed sequentially, but it isn't all that hard to figure out or do). Seriously, you should try it out, it's an incredible sim now with TF, the single player and multi are both fantastic.
Either way, BoS will be great as well, but if you want a Battle of Britain sim that is very close to what BoS is going to offer soon, CoD is now the one to fly, much unlike it's first versions/attempts/releases. It was truly awful from everything I read and saw.