CSGO or Face it

Debatable. Both have pros and cons.

FaceIt Pros:
-Smarter/more experienced/communicative team players
-Cheaters banned often
-Better tickrate iirc

FaceIt Cons:
-All the wannabes with the super good setups and internet are there. Unless you have a 144hz monitor or better, and a ping below 20 or 10, you ain't going anywhere.
-All the cocky esports wannabe kiddos are there too. Plenty of them with silent WHs, thinking they'll get taken into some esports team by cheating/looking "pro".
-Anyone can play afaik - those with free CS:GO accounts too. MM/Prime is now paid-only (as of a few months ago).

MM pros:
-Just the last thing I mentioned above in FaceIt Cons - Prime-only matches.
-Valve won't power-abuse you as FaceIt admins might. But that could be considered a con, since it includes Valve not banning cheaters that VAC doesn't detect. I know at least 5 people w/ Global Elite rank that use WH/Aim w/ 2k hours+ who are nevereven trying to hide the cheats.

MM cons:
-Lots of cheaters
-No proper team/rank balance.
-Bad tickrate.
-Chimp IQ teammates/trolls/griefers. Better have your own, premade 5-man team.
etc.

Just go with FaceIt, or if you're willing to spend some money to have way less cheaters, ESEA. ESEA>FaceIt>MM imo.