They are very different games. I was once enraptured with minecraft but poor recent inclusions (hunger, potions, enchanting) have diluted the once simple nature of the game. Terraria never claimed to be simple and it does complex so very well. They are, sadly quite different. If it's the rpg elements, the combat, and the monsters that excite you then Terraria's the way to go. If building and exploration are more of your thing then Minecraft it is.
Hint: If you select Minecraft, then download MCNostalgia and roll it back to 1.7.3 for the best of both worlds. New additions like pistons and dogs, while still retaining the essence of Minecraft. 1.7.3 is, in my opinion, vastly superior to Terraria.