The biggest difference is they published a paper on it, which I guess is good marketing ( I hadn't heard of proofofexistence.com). More substantively, they poo

The biggest difference is they published a paper on it, which I guess is good marketing ( I hadn't heard of proofofexistence.com). More substantively, they pool transactions and eat the cost, so it's free for the user. proofofexistence.com charges the user 5 mBTC, and you need to know how to send bitcoins to an address. Also they use Twitter as a secondary hash store and provide a somewhat better interface (you can directly upload hash in case you don't trust their javascript hasher or directly type text into a box, and they provide a git plugin as well).