So far in the poll people liked STEAM more, around 1 third for the Order, two thirds for steam.
Many say it's a choice between gameplay (STEAM) or visuals (the order). But I think there are more things involved. For instance artstyle is an important theme. The Order has a realistic looking artstyle with a smoky and somewhat dark historical London. Steam on the other hand had a decision for a comic-look (that is intensified in the cutscenes and the speech bubbles).
As a side-note: Rafie and Fededx, can you both please stop the derailing.