Battle and Strategy wise, I go with Gyarados. I always prefer offense over defense, unless the defense is something to take note of (like blissey or shuckle). This is the single reason why I would prefer Gyarados over Milotic (keeping in mind that their speed stat is equal).
Gyarados, equipped with the right nature, item and moveset, is one helluvan OHKOer. The only downslide in its case is it is too vulnerable to electric types, and its defense stats are not very noteworthy; which makes it susceptible to OHKO moves itself.