Yeah, as has been said quite a bit of times.
Minecraft is great as a sandbox game, to just bugger around in and see what you can build/do etc. Though I have to say, I enjoy minecraft more with certain mods than without. I basically built everything I wanted with hard work and all, but I couldn't be bothered continuing after getting diamond and making some good stuff, getting a nether portal... and... well... yay... that was it. so I just explore mods to have a better goal to work on in minecraft.
In terraria there is a clearer goal, there's bosses you can fight, there's places to explore where different stuff can be found and you need better gear to actually fight harder bosses. Also reading in the terraria updates there will probably be new zones to go to (which are harder and have better rewards etc.) it's less of a sandbox and more RPG-ish. Where the building has a clear function in the game and adds to the gameplay, but it's not about building the biggest stuff (though it certainly is a possibility).
eventually it comes down to what you prefer yourself.