Marvel Scale is just about always better for Milotic. It shouldn't be used as an all-out attacker, it should be used defensively. Marvel Scale boosts Defense when statused, which helps greatly with Milotic's already impressive defensive capabilities. It can also be easier to get the Marvel Scale Boost, as you can simply use a Flame or Toxic Orb to activate it. While Competitive can be good, it's generally a more niche option.
Note that Slowbro and Toxapex are generally better defensive Water types. Use one of those, unless you're absolutely dead-set on using Milotic for some reason.
Hope I helped!
