Github pages can only run builds of plain jekyll sites with no plugins. Netlify will work for pretty much any static site generator (right now we support ruby, node and python).
You can also trigger new builds without pushing to Git (essential for pages like https://www.staticgen.com that grabs data from somewhere else than the Github repo).
Apart from that there's hosting features like:
SSL for custom domains
Redirects
Rewrite rules
Fine grained header control
Lossless image compression
Automatically moves asset to content addressable URLs on a cookie-less domain with far-future expires headers
You can also trigger new builds without pushing to Git (essential for pages like https://www.staticgen.com that grabs data from somewhere else than the Github repo).
Apart from that there's hosting features like:
SSL for custom domains
Redirects
Rewrite rules
Fine grained header control
Lossless image compression
Automatically moves asset to content addressable URLs on a cookie-less domain with far-future expires headers
There's a few more things in the works as well.