Ruby/Sinatra used to be my favorite combo, but since Rails started going off in a weird direction (anti-js, asset pipeline, choosing AR over DM, insecure defaults pre rails 4) there has been less good stuff written in Ruby to draw from to augment Sinatra... Rails has become a rapidly changing but rarely improving API. That Ruby 2.2 breaks Rails 3.2 is just one more reason to look elsewhere.
I question why anyone would start a new project using Ruby at all. This is not a troll, I really don't think there are significant benefits anymore.