Since I should know...
For one thing, we're set up differently. FF.net has the capabilities to separate fics into different categories by genre, characters involved, rating, and so on. FF.net is also HTML-based, while we have BBcode.
FF.net also has different standards. Here at PFF&P, I try to maintain a standard that the normal members asked for, with both fics and reviews. For example, while at FF.net, a person can leave a review that doesn't say much, like "rit moar!", here at PC that's not really wanted, and easier to keep an eye on.
On the plus side for FF.net, you can reach a much wider audience than you would here. So if you want more people to read your fic, you can go there.
A benefit here is the community feel. The authors can get together and discuss their writing and ask questions. I don't know how it is over at FF.net, but the last I knew, that didn't happen a lot.
Other then the fact that the one on this site is focused only on Pokemon.
Does
no one know about Other Writing? It does exist farther down the forum.