There are various things you can do in ROBLOX that you can't do in Minecraft, as i haven't played the game i can't decide. But ROBLOX is a knock-off of Minecraft. That isn't necessarily a bad thing because the brand 'Oreos' was a knock-off of 'Hydrox' and in this case the knock-off was better than the original. In my opinion Minecraft is better, but i haven't played ROBLOX so i can't make a proper judgement
Well with minecraft it is a set game. You follow the game but some plugins make it more interesting, but it's still almost the same on each server. You go on roblox and each of the millions of games are different. No set rules in it or anything, just whatever the developer wants.