That's what you do on Roblox also... Not much difference really. It just depends what you like.

You only pay if you want Builders Club, a.k.a Membership. You can play as a free user.

Hm. From my understanding of Roblox. I've played since 2008. And I've never PAID real money for it. I've also quit for 1/2 year since I got bored of it and my account was still there. Also from my understanding of Minecraft, you have to BUY the game to play it. Unless you want the Demo or very old versions. On Roblox there are achievements but their called Badges. Also everyday on Roblox you will get 10 Tix or Tickets if you are a Non-Builders Club Member for you to spend on Items in Roblox. When you get a visit at your place you will get 1 Tix also. On Roblox you can basically build anything you want. There's objects like Wedges, Block, Ball, and other custom made meshes by Roblox themselves.

Okay first of all. It's easy to make something on Roblox than Minecraft. You do NOT need to add an mod into Roblox to get something from someone since you can transfer via Public Domain a.k.a Free Models. While in Minecraft you have to go into the .minecraft folder, and minecraft.jar to add and delete stuff. It's very very easy to mess up. Also Roblox doesn't steal things from Minecraft, because Roblox was created years before Minecraft was ever made. Not to mention the fact that in Minecraft you CAN'T program something without using an external editor. While in Roblox, the Roblox Studio program is equipped with Lua IDE. But I'm not saying Minecraft is bad or anything, I play both and I see the differences. What I like about minecraft is that it runs on Java. Java is a programming language that's more powerful that Lua. Java has been used in many different devices and programs. With Minecraft, sure you can't actually build whatever you want with the blocks Minecraft gives you like how Roblox does. But you do get a Generated world that's huge and won't lag as much as Roblox. But I wouldn't support Minecraft in this argument since to me, programming is more important than building and Minecraft can't let you program without an external IDE so I support Roblox.