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.