Feathers vs. Sails
From a feature standpoint, Feathers and Sails are probably the most similar of the comparisons offered here. Both provide real-time REST API's, multiple DB support, and are client-agnostic. Sails is bound to the server whereas Feathers can also be used in the browser and in React Native apps. Both frameworks use Express, with Feathers supporting the latest Express 4, while Sails supports Express 3.
Sails follows the MVC pattern while Feathers provides lightweight services to define your resources. Feathers uses hooks to define your business logic including validations, security policies, and serialization in reusable, chainable modules, whereas with Sails, these reside in more of a configuration file format.
Feathers supports multiple ORMs while Sails only supports its own Waterline ORM.
Sails allows you to receive messages via websockets on the client, but, unlike Feathers, does not directly support data being sent from the client to the server over websockets. Additionally, Sails uses Socket.io for its websocket transport. Feathers also supports Socket.io but also many other socket implementations via Primus.
Even though the features are very similar, Feathers achieves this with much less code. Feathers also doesn't assume how you want to manage your assets or that you even have any (because you might be making a JSON API). Instead of coming bundled with Grunt, Feathers lets you use your build tool of choice.
Sails doesn't come with any built-in authentication support. Instead, it offers guides on how to configure Passport. By contrast, Feathers supports an official authentication plugin that is a drop-in, minimal configuration, module that provides email/password, token, and OAuth authentication much more like Meteor. Using this you can authenticate using those providers over REST and/or sockets interchangeably.
Scaling a Sails app is as simple as deploying your large app multiple times behind a load balancer with some pub-sub mechanism like Redis. With Feathers you can do the same but you also have the option to mount sub-apps more like Express, spin up additional services in the same app, or split your services into small standalone microservice applications.