This post is coming from an unbiased standpoint.
Water types are MUCH better than Psychic-types. As stated above, waters only have two weaknesses: Electric and Grass. However, the only usable offensive grass attacks are Giga Drain and Hidden Power Grass, while good Electrics are few and far between. The only types that resist Water are other Waters (lol) and Grassers; as with Electrics, there are only a few good grassers. They generally have the defense to stand up to neutral attacks from powerhouses like Salamence/Metagross, and can Surf/Ice Beam these Pokes to oblivion. Waters counter a lot of overused Pokemon, so they are always welcome spot on the team.
Psychics have gotten the short end of the stick. More overused Pokemon, such as Tyranitar and sponges, can wall psychic attacks. Shadow Ball is used on several physical sweepers, which will send Psychics to their doom quickly. Generally, the only Poison/Fighters in overused are Heracross, Gengar, and Weezing, while the latter has sort of died out from what I've seen. They are generally harder to fit on teams since they counter less Pokemon than waters. Most Psychics need another type to be truly efficent (i.e. Steel on Jirachi helps it resist Normal and Rock, Celebi's Grass-typing helps it resist Water attacks), as pure Psychic is only an okay typing.
THIS IS WHAT HAPPENS WHEN YOU HAVE TIME ON YOUR HANDS!