Milotic is awesome! But Gyarados is can learn a better and more varied move set. My Milotic uses the old Toxic and then Recover on any opponent Pokemon but Gyarados can learn Thunder. They both have great Defence and Speical Defence as well as Attack and Special Attack. Speed is also good but it is all about the Electric move I think.
I love Milotic more than Gyarados. They both are very useful in their different ways. But Gyarados is stronger then Milotic but only because of the various moves it can learn e.g Thunder, Thunderbolt, maybe Solarbeam I can't quite remember.