Terraria's gameplay is more fun moment to moment, and it's structure is better.

But Minecraft is more powerful and offers a lot more than Terraria in terms of gameplay, especially when you consider all the mods and redstone.

Terraria has the best combat as it is very fast paced and frantic. Its 2D nature makes creating things feel lame.

Minecraft has the opposite problem: its 3D nature allows for the creation of great structures. Its combat, however, sucks as it does not feel meaty or interesting.

Both have some modding potential, though Minecraft wins in that regard due to the countless modpacks out there that include up to hundreds of mods that are harmonized to work together.

The mods in Minecraft were so substantial that they helped give rise to games like factorio.