Roblox or Minecraft: which one allows for more creativity?

Minecraft, you're limited to your imagination. With Redstone, you can make anything you want, people have made working computers in Minecraft

Minecwaft

From my experience, Minecraft is easier to jump right into and start doing stuff. I've spent most of my childhood on it anyway. Roblox is fun, but in terms of making your own stuff, it's going to take a while to get used to. Roblox is great for playing other, custom made games though, by other, more skilled users. But to answer your question, Minecraft would be my choice.