Well they give different things, Satan and Thanatos being fast at burst building because of their combo atk rate buff, and Satan also has that overdrive reduction which can save you in your time of need if you want to delay the overdrive for 1 more turn, while Amon is more of a boss killer offering high damage and an extended stun that can help a lot in certain situations like ragnaroks and stuff
In the end I guess its all up to preferences.
Personally I like Amon Unleashed more because of what she offers (right now and in the future) and obvious waifu reasons lol.