When Terraria came out it looked much more like a finished product than Minecraft. Terraria was perhaps easier and faster to make as it is 2D, this leaves much more time to put new elements into the game to make it more like an RPG and less like a ripoff of Minecraft. When Minecraft was released for download it was in an alpha state and is still a few months and many updates away from full completion. When Terraria came out it seemed like a finished game that would receive updates regularly to enhance the gameplay. This will most likely happen when Minecraft gets its full release. So, for the moment I like Terraria more for the single-player and Minecraft more for the multiplayer but my views may change in the months to come.