Cofagrigus outclasses all the way
Cofagrigus works as a monster physical wall and a decent special one at that too. It has trolly moves such as will-o-wisp and pain split. It can also function as an offensive trick room sweeper due his access to nasty plot and great bulk. Bannette on the other hand while having good offensive stats, has pitiful bulk and speed and a pitiful movepool along with that. The best option you have is a suicidal trick room, and that's already outclassed by cofagrigus. Competetively, you should pick cofagrigus.
Once again Cofagrigus wins. Unless you plan on sucker punching Bannete is just gonna die to much ingame to do much of anything. Cofagrigus can play a defensive role and play a little offense too(a.k.a tank). The fact it can learn mean look is also very helpful for catching roaming legendaries.
Cofagrigus pretty much outclasses Banette
But there's a twist...
I forgot about this but Banette gets a mega-evolution which gives it a LOT more toys to play with
Mega Banette can play an offensive prankster or one with trick room. Banette gets its attack up to a massive 165 (though it doesn't get items which kinda sucks). It can burn with will-o-wisp if you want it on your set, but a guarenteed trick room is pretty dangerous because once that's up, Banette gets to use it's attack and most powerful moves to it's full potential. It also has priority such as sucker punch and shadow sneak. Overall Banette get's a lot better with it's mega evolution than before.