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Minecraft is much more fun to dick around in, where as Terraria is more progress focused.
You can fuck around and set stupid objectives like "Blow shit up with TNT" or "Go exploring" or "Troll my Friend" or whatever and still have fun in Minecraft. You don't need to work out what the game wants you to do to have fun.
In Terraria the fun of the game comes from actually doing things that'll progress you, or the game, forward, such as finding rare loot, beating bosses, getting NPC's to come in ETC. which is fine if you enjoy that, but it means that the girls actually need to plan out their progress and work out what the game expects them to do next, which is not especially good for stream content and leads to a metric fuck ton of back seating from frustrated terraria vets who've been playing since release.
tl;dr Terraria actually makes you work for your fun from the outset, where as Minecraft just lets you make your own wherever you find it. Neither approach is better or worse than the other, but the latter approach is a lot better for streams, especially from a group of people who are largely not traditional "gamers"
