Hey,
I just checked Steam and Origin for their preorder on Tomb Raider. All the prices are in Danish Krone (DKK) you can do the conversion to your own coin :) Anyways, Steam lists Tomb Raider Survival Edition at 389 DKK and Origin lists the Survival Edition at 299 DKK.
As you can see, Origin is about 24% cheaper! Now on Steam you also get The Guardian of Light, which doesn't matter to me since i already have it. You get a Single Player DLC which looks to be very short since it from the description only takes place on 1 crashed plane. Still pretty nice i would think. And then you get some Multiplayer DLC, which i don't care about since i don't play Tomb Raider for the Multiplayer. So for me, i get 2 items i don't care about, 1 i do care about, but is extremely short. So basically i would pay 24% more just for the option of having an extra plane in the game. That isn't enough for me. Origin wins this round for me :P
Links:
Steam: http://store.steampowered.com/app/203160/
Origin: http://store.origin.com/store/eaemea/da_DK/pd/productID.273547700