I completely disagree. Rails takes care of the foundation for you, and a novice doesn't need to know how it's implemented for a while. It also has good practices built in like dev,test,prod environments, testing infrastructure, mvc separation, etc.
Sinatra assumes you will roll all of this on your own. A novice won't know to do this, or how to do it, so I think it's better to have it done for you at first.
I mean it's nice that rails can generate an "hello world" app without writing a line of code, I think you'll learn more about programming and writing a web app if you can better see what's going on:
Sinatra:
get '/hi' do "Hello World!" end
Rails:
rails generate controller hello
For that matter, don't let all this web and database stuff scare you off. Ruby is useful and powerful as a stand alone scripting language. You'll learn a lot just writing scripts that can run on the command line (and that will lay the foundation for the web stuff if you'd like to dig deeper). The web stuff isn't really any harder, but it's another layer of things to learn.