I would say go for the direct purchase from MS.
This is one title where buying from Steam has no real advantage. Most of the game files (100Gb+) and maintenance patches are downloaded from the FS/Asobo server, not from Steam - which is why the current annoyance over file looping and having to throttle speed to 500KB/s to prevent it. While this huge download interspersed over several days is going on, Steam thinks you are playing FS so you can't run any other title in the meantime or use your Steam account on a different machine.
Steam also seem to have stooped their no quibble approach to refunds - I tried a couple of days ago being totally fed up with the d/l situation and it just got knocked back.
Thanks for the reply, I never thought of Steam being tied up while it downloaded