which database suits my application mysql or mongodb ? using Node.js , Backbone , Now.js

Question

I want to make an application like docs.google.com (without its api,completely on my own server) using frontend : backbone backend : node

What database would u think is better ? mysql or mongodb ? Should support good scalability . I am familiar with mysql with php and i will be happy if the answer is mysql. But many tutorials i saw, they used mongodb, why did they use mongodb without mysql ? What should i use ?

Can anyone give me link for some sample application(with source) build using backbone , Node , mysql (or mongo) . or atleast app. with Node and mysql

Thanks