Though the motives are different, both manga feature a man and a woman who play detective in order to find serial killers. Goth is deeply psychological with a dark atmosphere and creepy scary. Fraction is psychological and borders on guro. The detective work is sound and the result is shocking. The author's narrative intervention in Fraction is a fresh take. Fraction makes you look at the principles of crime fiction. Both are disturbing in their own way.
Both seinen manga have their mangaka inserting themselves into their stories to further the plot. Both works are fairly meta in nature. But do keep in mind that while Opus has a strong shounen feel to it, Fraction is an out and out gore/horror manga.
Sex and violence. Serial killers and gore. The leads of both manga promise sex as a way to lure their victims. Both get off on killing the opposite gender. Ana Satsujin feature a murderous woman, while Fraction features a murderous man. The belief system behind each killer is insane. Each story is quite shocking and ridiculous (in a good way). Neither are afraid of being self-aware. Fraction is far more explicit and is full-blown guro.
The premises of both these manga are very different, but the treatments of their subjects are pretty similar - both have gore, shock, twists and turns, and a healthy sprinkle of psychological jazz in them.
Note: Fraction has a lot of gore in it, so if such stuff triggers you, avoid it.