i really dont know, both games are good in their own way...
Minecraft in its quality of content, Roblox in its diversity of content
basically depends on how both games are viewed.
minecraft normally has a straightforward gameplay but lets you do everything you want easily
while roblox lets you do anything you want at the cost of it being complex, forcing you to acknowledge things like scripts and tool stufs, etc.
roblox is good in its own ways,but minecraft is good too.
Two different genres, very hard to compare. Just play both.