Well like your alakazam or gardevoir question they both have differant pros and cons.
Milotic has high hp and S.defense and a good S.attack stat a good special tank. Milotic is also good for walling with high hp and S.defense and moves like aqua ring and recover.
Gyrados is completely differant and also has flying as his type. He speciallizes in attack with good S.defense and speed though becuase of the common thunderbolt he is usually a sweeper instead of tank. he makes a better sweeper becuase of dragon dance as well as strong moves like waterfall and earthquake.
Milotic has a limited move pool and can be out stalled by other bulky water types. Becuase it's best attacks are ice beam and surf. And it has trouble covering electric types though is otherwise great.
Gyrados has the unfortunatly average speed stat of 81 thus lowering sweeping potential. One of gyrados's worst and most famous drawbacks is it's 4x weakness to electric. Gyrados has not very good defense so it can be easily koed by spark, volt tackle, [wild bolt], and stone edge.
So if you want a tank or wall go with milotic and if you want a sweeper go with Gyrados