AO3 is probably the better site in general, but it won't hurt you to cross post.
EDIT: Assuming it's a fanfic of something that does well on RR. A Twilight fanfic? Probably not. I wouldn't waste my time. A litRPG anime fanfic? Yeah go for it.
But it depends...
Some works have very small, or at least inactive, fanbases, and thus fanfiction will receive less attention even from fanfiction dedicated sites.
Someone_77 Wrote: Basically the title.I think fanfics are well recived in general, if they are the more realistic approach to the world. The only problem being that your audiance is restricted to the type of person that knows the work.
For a good example look at born of caution. Everyone played atleast one pokemon game, but the novel dosnt expect you to be throat deep in the lore swamp and know every single character or item.