As a parent who's kid is into both, if there's a choice, I'm pretty sure I'd recommend Minecraft; In Minecraft, the default vanilla gameplay itself is quite intelligent and creative. No such guarantee with Roblox, where, depending on the game, there's a decent chance the kid will get sucked into endless mindlessness.
That being said, I'll be taking a look into Roblox for scripting/programming, I haven't done so yet and I'm all about teaching her how to "cheat" -- and actually, this is a place where Minecraft feels like it has faltered a bit, or I'm looking in the wrong places -- I had a great deal of success teaching other kids (I run a nonprofit) Python with the "MCPI" api, and I can't seem to see if that's been updated or if there is a modern equivalent?