In terms of how the games are operating? I would say neither - the end user is the one who has it worse since they need both uPlay and Steam to be on to play one single game......
I mean in regards to which company would take the bigger hit from it. Would it be Ubisoft for making their games digitally available only through their own Uplay service, which no one seems to like, or Valve for no longer be able to offer Ubisoft titles?