Slowbro is a classic tank. While he does wonders on trick room teams, he is also a solid set up Pokemon.
Milotic is more of a one trick pony. Functioning best with a choice band, it also has a place on rain teams. It has fewer weaknesses than Slowbro, but negligible bulk.
It is hard to say which is better, as they both fill different roles. If you are simply looking for a Pokemon to help you through the story, though, Milotic is your best bet. Post game, Slowbro is generally a better choice.