How is Rails different from Sinatra? Why is it more widely used than Sinatra?View other answers to this thread
If your app consists of fewer endpoints and is RESTful you should probably use Sinatra. Rails is a framework focused on writing model driven web applications, but Sinatra is a library for dealing with HTTP from the server side.
Rails is a framework that focuses more on developers' happiness which leads to more adoption among software developers. Also big websites like GitHub, Heroku, Twitter have adopted RoR. This may be one of the factors that encourage new developers to start learning RoR.