I would get both. Minecraft is like building with virtual legos and Scratch is learning how to program.
I tried Scratch and DD got into it for a few days, but they have been playing Minecraft for years. DS was around 3 and now he's 6 and they are still both really into it. If you choose only one, do Minecraft (but be prepared for obsession).

There is a free version of Minecraft on the PC. My kids always play on the PC but we have a paid version now.