Both are easy to setup and are accessible by anyone using "inproxies" such as onion.to or i2p.us.
Another advantage is that you can host the sites anywhere even behind a firewall or a NAT as long as the computer it's hosted on can run Tor or I2P.
I personally have a preference for I2P, since this is its main purpose while Tor's hidden services are not the primary purpose of Tor (which is to anonymize users on the clearnet).