The biggest difference is, of course, that you are purchasing a brand new smoke alarm with Nest that is significantly more expensive and takes more time to install.
Wtth Roost, you are simply upgrading your existing smoke alarms! Nest Protect and the Roost Smart Battery both will alert you if your smoke or CO alarm sounds. However, installing Roost is as simple as replacing a battery in your smoke alarm. You don't need any tools or complex installation that requires you to hire someone. Roost Smart Batteries will also let you know when the battery level is low so that you can avoid those 3 AM chirps. All for an affordable price that gives you peace of mind without breaking the bank.