I myself prefer Seismitoad, but that's because I don't really care for the elemental Aipoms. Both of the pokemon are good, but neither of them are truly great. They both do roles that other pokemon can do but better. Seisitoad is essentially a weaker version of Quadsire/Gastrodon being the Ground/Water tank or wall he is. Simipour is basically a weaker Starmie without the Psychic and Rapid Spinning.
In a 1v1 battle between the 2 pokemon, Simipour will win simply because of HP Grass, but as overall pokemon, you can't particularly claim one is betterthan the other because they do completely different roles and have different capabilities for different strats.