Which should I buy?

Valkyria Chronicles is 3d turn based sort of game where you make plans and destroy tanks and such,
Valiant Hearts is 2d adventure story sort of game with pretty interesting story and nice graphic.

So either you prefer 2d Adventure or 3d Strategy game. Both games are pretty good. Choose what you feel like.

I would go with Valkyria Chronicles.

Valkyria Chronicles, I think Valiant Hearts will be in a future bundle (my opinion)