Competitively, use competitive on Milotic
There is really no reason to run gothitelle other than with shadow tag. Shadow Tag allows gothitelle to trap Pokemon and then stall them with movesets like cosmic power plus stored power. Gothitelle is more defensive than milotic, so running a defensive set makes more sense.
Competitive on Milotic, meanwhile, makes a lot of sense. Intimidate and moves like mystical fire (sometimes) and Max moves are common in all formats, as well as icy wind in doubles, making Milotic able to boost its special attack and become a powerhouse. You can use it when the opponent is least expecting it. Otherwise, with milotic, you can go with marvel scale, an ability that boosts defense when inflicted with status which with a flame orb is fine or cute charm which gives a chance when hit with a direct attack to make the opponent infatuated, making it luck based.
Milotic is neither defensive or a luck based Pokemon (like machamp with fissure), so yeah don't use either of those. Definitely pick Milotic. You can even have both on your team, just don't use competitive gothitelle.