While i enjoy both, i have to side with Minecraft. Ive played it since before doors were added, and i havent stopped playing it since. And while i did play terraria for a long amount of time, and collect every piece of gear, and explore every nook and cranny on my favorite world.
Minecraft is still addicting. I still play it regularly. I havent touched Terraria in months because theres not much to go back too. Without the 3rd dimension minecraft has, your buildings just arnt as.... deep. Once you have everything, and youve built a grand structure thrice over, it just kinda falls off. It was an amazing ride, but it had to end.
Minecraft is still being developed too. Were seeing newer, and better features nonstop. And even if its not as exciting, or addictive as terraria once was, it still draws me back. They are both amazing games, but Minecraft just lives on too me. Terraria was great, but Minecraft still is.