I have a harder time using Sinatra because I never know where to put things.
Also, the biggest problem with learning Rails is the Ruby language. After you learn the language, switching from Sinatra to Rails and back is not problematic.
Yeah, Rails gives beginners a big advantage by generating a huge scaffolding. It clears up a lot of ambiguities about where things go, and as a beginner it lets you explore different things that you can do in Rails.
As a novice developer, I absolutely loathed scaffolding (IDE-generated header and source files) because I could never understand why all the code got there, how do those files relate and what does it do.
I tried to learn COM once but gave up essentially because I could not find any docs not falling back on IDE code generation (and that code was huge and absolutely opaque).
I always loved tools which let you to type in a small program from the scratch and run it. So, perhaps, sinatra is the way to go for beginners.
I didn't find the scaffolding that difficult to understand, because I've used CakePHP before, but what gets me in Rails is the magic helper keywords. For instance: ActiveRecord.find_by_columnname(value), {controller}_path, {controller}_url.
I'm fairly certain that's not all of them, but I don't even know what those things are called to look for them. Those 3 are covered in a lot of tutorials without really explaining the full list of stuff Rails does for you automatically.
I guess everyone is different since scaffolding always scared me when I was a beginner. It felt like entering an existing code base not knowing where things should go and what you are allowed to edit, except unlike a real code base none of the functionality is there making it even harder to figure out where to put things.
I have always preferred to start simple and localized and add complexity as the project goes. Especially when I am learning something new.
Also, the biggest problem with learning Rails is the Ruby language. After you learn the language, switching from Sinatra to Rails and back is not problematic.