I made a similar leap from C/C++ and C# without much web development a year ago. I have since developed several projects using both Rails and Sinatra using both ActiveRecord and DataMapper for ORM. The Sinatra/ActiveRecord combination is my current default.
In general, ORM pain decreases significantly as soon as you go with a dynamic language like Ruby or Python. Hibernate nearly made my head explode the first time I saw it.
In summary: I would recommend Ruby, Sinatra and DataMapper as a starting point.
I second Sinatra+Datamapper. I'm not much of a web developer, but I really love the simplicity of Sinatra. Using it is like C, in the sense that it provides very little for you (ie. it stays out of your way).
Unlike Rails, there really aren't many conventions to follow, and things don't seem to break between releases as they do with Rails (from its constant and exstensive evolution).
I'll also throw in a recommendation for using HAML if you're building any HTML for the client side.
Padrino is also at a nice waypoint between rails and Sinatra. I recommend Sinatra for hacking out the basics of an MVC app then switching to Padrino once the generators make sense.
In general, ORM pain decreases significantly as soon as you go with a dynamic language like Ruby or Python. Hibernate nearly made my head explode the first time I saw it.
In summary: I would recommend Ruby, Sinatra and DataMapper as a starting point.