Okay, I actually like both. You can earn real money by developing games on ROBLOX, but it takes a lot of time and you'd most likely be paying other users to help develop your games. Unfortunately, it's very difficult (if not impossible) to get involved in ROBLOX's market without spending real-life money. ROBLOX is very versatile and players are always finding ways to change the game, but for some reason, those darn tycoons still exist and they're some of the most popular games...
Minecraft is more about survival and building, which appeals to a different audience. You can build in ROBLOX, but most of the time it's just to build maps for games, and players don't usually build directly in the games they're playing. Minecraft allows you to join countless servers hosted by players in the community, and similar to ROBLOX, players are always finding ways to change the game. You can also add mods to Minecraft from a nearly endless pool of mods created by the community, so you can always try something new in Minecraft.
I've definitely spent more time playing Minecraft than I have playing ROBLOX. I enjoy both of them but I find it's easier to make friends in Minecraft because I might be playing a minigame with someone on ROBLOX and then never see them again unless I add them to my friends' list.