I agree that Terraria has more game elements but it's far less compelling as a sandbox to build things. Constructed environments can't be as complex or interesting in two dimensions, and of course this is nothing like redstone circuits in Terraria.
Also as far as the content in Terraria, it doesn't really have THAT much. There are only 3 bosses, all of which can be soloed without the top gear (better gear makes it easier but you don't need the best in the game to kill them). I got kind of bored with just loot farming when I already had good enough items to defeat everything in the game. Even so, it's a lot of fun and easily worth the ten bucks.
With Minecraft I didn't find the combat to be interesting. In fact I found it annoying to have to stop whatever construction project I was working on to go hide inside for the night or risk having a creeper blow up my building. When I want to concentrate on building stuff, I usually turn the monsters off. The "game" portion to me is less interesting than the sand box play.
I love both Minecraft and Terraria but for different reasons. Despite some similarities they don't play the same and each game has its own strengths.