It's hard to compare the two. It's hard to evolve Feebass, but you can make it stronger with TMs. Mgikarp, on the other hand, evolves through level up, but you can't teah it TMs, AND it doesn't learn any damaging attacks until level 15. Gyarodos learns different attacks than Milotic, Milotic learns different attacks than Gyarodos. It's hard to compare the two, but I go with Gyarodos because it's from the older gen. :D