This purely depends on which platform you own the most games on.
For me, I own like 4 games on Origin and 100 on Steam; so for me it would be Steam. Also, in terms of DLC I consider the Steam platform to be much more organized and tidy looking.
I have Steam open all the time, whereas Origin is only open when I want to play those select few games.
It is totally up to you, but that is my recommendation.