You have to define your terms. Antifa on the left is quite extreme but SJW is a vague term and may not even include them. Feminists are hardly likely to use violence against persons. Aren't they SJW? Alt-right might use violence but usually seems to be crazies who actually do.
Within the connotation of "alt-right", ideas of extremism are much more prevent. In comparison, "SJW" just brings ideas of extreme social ideology. A fairer comparison of connotation intensity to "alt-right" on the side of the left would be Antifa.
I think the real-life impact of the "alt-right" is a lot bigger than that of "social justice warriors". Leaving aside that I can't remember any "social justice warrior"-run massacre but plenty alt-right ones?
Two different kinds of extremism: u-/dys-topian vs. ethno-nationalist. I think the SocJus folks are potentially very dangerous *if* they get hold of political power at scale. The Alt-Righters are IMO much closer to having overt political clout & so pose a more imminent danger.