Hojo is the root of all evil in FFVII.
Sephiroth and Jenova were just more of his playthings.

But between these two, I have written a previous threat about how Sephiroth's character during the game is completely inconsistent and nonsensical when you factor in what we saw in Nibelheim. He has entirely different goals and no real motive. As such I prefer to think Jenova warped his mind in some way or another as there's no other explanation for his "I want to take the Planet back for the Cetra/Jenova" changing to "Screw the Cetra, I wanna be a God!"