actually, they're both, from what I've seen of them, manaphy has almost the same size as the other three, a similar movepool, but it has less base happiness and has a baby (which just makes it more special)
Although sheimi has everything similar to the other three pokédex-wise, it is VERY short and can't float
I think manaphy is the 4th gen mew, and sheimi is just an extra 4th gen mew, they both have 100 of every stat, which only mew/celebi/jirachi had up until now, so they should be categorized together, but seeing how manaphy already got its movie, shows some psychic powers and can float, it should be the new mew. Sheimi on the other hand can also be considered a new mew, but, well, the 5th one then (being at the end of the list and all makes it seem more-mewy)
I think this was just a response to the popularity of mew, celebi and jirachi as a Pokémon.