Hearthpwn

In a midrange/control shaman deck, including jade shaman, elemental shaman and control shaman, these 2 cards are always being included.

If given that you only have 2 card slots left, which which one is better to go? And what's your rationale?

stonehill to get earth elemental and/or white eyes all the time.

Stonehill Defender seems like the obvious choice to me.

By design, class cards show up more frequently than neutrals when you play a Discover card, and Shaman has access to some of the best Taunt minions in the game, including two legendaries.

So with Stonehill Defender you can expect to frequently see: Al'Akir the Windlord, Thing from Below, Earth Elemental and White Eyes (which actually gives you two extra cards instead of one becauseThe Storm Guardian).

The only class that can rival that is Paladin, with 3 legendaries that have Taunt. This explains why you see Stonehill Defender in a lot of Paladin decks.

Why not both? 

Stonehill belongs in every deck. Aggro, midrange, control. It's the new Drake.

I usually like to run both if possible. They are probably the two best defensive 3-drops in the game. Tar creeper is better for board control but stonehill is better for value. I'd use both as Tar Creeper is easily the better card against aggro when stonehill is easily better against control. If you can only run 2, I'd suggest either adding 1 and 1 or just looking at the meta and using it as a tech choice.