Gist is associated with your github account. Gist allows public and private pastes. Other gist users can clone or fork your code using a url that is created for each paste.
Pastie doesn't require a login, so it's nice for quick-and-dirty. Pastie claims to have a bot that you are supposed to be able to access right inside IRC. You send it a public message, it private messages you a link, you click the link, paste your code, click OK and the bot is supposed to announce the link to your paste to the channel. Unfortunately, I'm told it's never worked.
Pastie does have a nice feature for Rails developers
I plan to stick with gist for now, because of its connection to guthub.
EDIT: I didn't realize that gist not-only has syntax highlighting for lots of languages, it will auto-detect based on the filename extension that you give your paste (for example, this partial that I will discuss in my next post.) Now I'll stay with gist for sure.