Satsui no Hadou vs. Devil Gene
The Surge of Murderous Intent or the destructive "Devil blood"?
POWER
The way I understand it, the Satsui no Hadou is just complete drive with lack of restraint. It does not offer any more power than you yourself already have. Anyone could do it if they had the desire.
The Devil Gene however, is a family-specific trait to the Mishimas. Except for Heihachi of course. It seems to me to be activated by the need for power. As in Kazuya climbing up the cliff and Jin needing to escape from Heihachi. However, calling it unnatural might be unfair. It's part of Kazuya and Jin afterall. But however, it's not obligatory to use it. But Kazuya mad ea conscious decision to use it and Jin's was more instinctual.
EVIL
But as for which is more evil, those with the Devil power seem compelled to destroy. Angel kept Kazuya as just a brutal importer of animals and a tyrannical head of the Zaibatsu but as of T4, he seems to be completely consumed by it. As according to his T4 Prologue, he wants to master it and has probably cast aside all good in this obsession.
Jinpachi has his natural good side corrupted and overpowered by the Devil's desire to erase all life. Devil Jin seems to want to absorb Jinpachi's power and nothing else. There's no real glimpse of what he intends to DO with the power but I guess the ending would give a hint that too wants to destroy everything. However, Jin tapped into his Devil power passively a few times, just growing wings and not really being corrupted.
The Satsui no Hadou on the other hand is just the desire to make one a better fighter and win. Sure Ryu wanted to win and nearly killed Sagat but he wouldn't go around destroying everyone else too. Akuma, the firm master of this power, does not kill and can even hold himself back. He has no longing for pointless destruction, only to get stronger.
I'd say the Devil wins hands down here.
Which would win in a fight?
It depends on the user. As I said, the Dark Hadou can be used by nearly anyone. The strong and the weak. There is no really weak Mishima so they're just adding demonic power to their own immense ability.
But of those who have canonically used it, Akuma and Ryu, it would seem to be lower than the Devil Gene. The fact also that the Devil Gene alters your body to give you new powers is a further plus over the Murderous Intent. You have to master techniques still and arent' just handed nifty laser beams.
In conclusion, i view the Devil Gene as a powerup with far more evil and more power than the Satsui no Hadou. But as I stated, it really does depend on the user.