Hapi's philosophy is that configuration is more important than code. This is especially useful for very large teams because it helps developers maintain consistency and reusability throughout their code.
Hapi was developed and is still being used by Walmart. Being backed by such a major company means that it will not lose support any time soon and most importantly it's being developed by professionals and that you will always get support for it.
Hapi seems to be made with large applications in mind. The sheer amount of boilerplate code it requires is simply not practical for a small web app. This also means that there are few examples of Hapi applications around for beginners to learn from.
Sails follows the Convention over Configuration philosophy. This is shown even through the CLI that comes with it which quickly and painlessly generates and scaffolds a new project following the conventions already established by the Sails community.
One of the major benefits that Sails has is performance. But it's not as efficient when starting up. sails lift takes quite a lot of time to execute completely (although it is doing a lot of things behind the scenes - if you run it with the --verbose option, you can see all the different things
The Waterline ORM is a bit limited. Associations were finally added after much time and many requests. However, they are (as of 2015) still incomplete. It is not a great ORM for MongoDB. You will very likely be using native MongoDB queries throughout your code more than you might like (for example,