We've used express in mern.io, as it is, arguably, the easiest way to get started with NodeJS development.
Just as ExpressJS wraps around basic HTTP libraries provided by NodeJS, to enable you to manage things like routing, sessions, etc... in an easy way; SailsJS wraps around ExpressJS to provide you an abstraction level higher — giving you an ORM to work with a wide gamut of NoSQL, and SQL databases, real-time functionalities, auto- generation of REST APIs, and other fancy stuff.
The way we've structured the mern.io boilerplate, you would have to do a complete re-write of server setup, and database handling — we use mongoose as the ORM.
Propbably @somu can expand on this answer.