Both do, but it *is* sexist. It is far more widespread and vicious towards women. Ignoring that is not helping.
I'm not convinced that it's more widespread and vicious towards women, but if you have evidence I will gladly look at it.
I would posit that it's not at all more vicious than attacks on any other groups, but rather that it's nearly impossible to discuss "feminist issues" on the internet rationally. If you're a troll and you're looking to get a rise out of people, using sexist language is a great way to escalate an argument.
Also, it seems that for a person who identifies as belonging to a given group, and a troll starts spouting derogatory language pertaining to that group, it's written off as trolling. That is, any group aside from women. Any derogatory language pertaining to a woman's gender is taken as bigotry, harassment or threats.
It seems contradictory to me that many people who call themselves feminists claim that want women to be treated equally to men, but think they can accomplish this by shielding women from the realities of the internet.