I love both Sinatra and Rails, for different reasons. I've heard a few different heuristics for which framework would be better for your application, but I'm not sure the answer is all that simple, really. Regardless of which is correct for your application, I haven't seen a lot of people discussing how to move between the two frameworks.
I've done this move a few times now, so I wrote a blog post about the process you'd take to move a project from Sinatra to Rails.
January 17, 2012