According to the ESRB inFamous was rated T for Blood, Drug Refrences, Language, Suggestive Themes, and violence, while Prototype was rated M for Blood and gore, intense violence, and strong language. The language in both of them were the same according to their droid app. And they bother to point out that in prototype you can consume people by absorbing them into you, but they don't mention you can do that in inFamous(its called bioleech instead). They mention that prototype has scenes of crows eating dead bodies, while inFamous has a scene where the police open fire on innocent civilians trying to escape the horrors in the city, i don't know bout you but the second seems way worse. Also as they said in inFamous Sasha is a naked chick covered in tar talking about how she had sex with someone, I can't remember who but I haven't played in about a year. To me inFamous is a a lot worse but I guess the ESRB knows best, even if they don't actually play the games. Yes I am serious, which seems incredibly ridiculous as anything will seem a lot more horrible, violent, and vulgar if you don't actually experience the entire thing as opposed to just seeing the most violent, vulgar and inappropriate parts.
TL;DR the ESRB doesn't actually play games, look it up.
OT: No that isn't fair, it should be determined by whoever rates video games in your country, that would be as random as me, an American citizen, having to abide by the laws in England, when I'm not even in England.