SailsJS is a full-featured framework with built-in tools to deal with business logic (models, services, ...). It comes with an opinionated architecture too. While ExpressJS is a lightweight framework on top on of the standard library. Itallows to set up a http server and to handle requests through a bunch of functions (named "middleware" for general purpose and "controller" for a specific route).
SailsJS is more suited to build a full-featured web application (it's closer to Ruby on Rails). ExpressJS works better for micro-services, prototypes or if you want to have an architecture of your own (it's closer to Sinatra in the Ruby world).