First you can have a look to these related questions :
http://stackoverflow.com/questions/81830/rails-or-grails
http://stackoverflow.com/questions/5087/learning-ruby-on-rails-any-good-for-grails
http://stackoverflow.com/questions/1283935/what-technology-asp-php-joomla-rails-grails-for-a-website-from-scratch
http://stackoverflow.com/questions/397228/is-grails-worth-it
http://stackoverflow.com/questions/2055396/is-grails-now-worth-it
Now, I will try to answer you according to your requirements you have communicated and the information I have gathered from the internet and my own experience.
Ruby on Rails
I do not advice you to start with RoR because you are a Java developer and you will have to learn a new language (Ruby) and a new environment (Rails). The hosting issue is not a real issue. You can have a VPS hosting plan for $10 (www.enjoyvps.com) perfectly suited for small grails app. If you application needs more memory, you might need to add another 10 Bucks.
If you hosting is really THE critical factor, go with Python/Django or PHP/Kohanna (a very good MVC framework). Otherwise, according to your background, Grails is more suited for you than Rails.
Grails
Few months ago, I had the same dilemma as yours and I decided to have my way with Grails. Why?
Because it's cool !! I mean, community is very helpful and dynamic, Groovy is a pleasure to develop with (be careful : thereafter , there are good chances that you will loath Java). Also, it is a state-of-the-art framework based on very-well established technologies (Hibernate, Spring, Java) and hence, it can improve considerably your market value as a developer. Grails is my favorite
Spring Roo
Roo is Grails for Java. So if you don't want to learn Groovy and if you need pure Java application (that will always run faster than a Groovy-based app), go with it. The community is smaller than Grails but the project is supported by SpringSource who is quite active in the community. I think that if you want to go as fast as possible, this is the solution for you.
Spring Application
You should choose this option only if you don't like Spring Roo integrated technologies (Hibernate, JSP, Maven...) and you want Java absolutely. Otherwise, there is no interest (except for educational purposes) of spending hours of configuration and tuning when you can build sophisticated enterprise applications in a best-practice manner within minutes (with Rails, Roo or Grails).
Each of the options above will provide you enough performance for the application you want to build. A lot depends on best practices for website applications like good architecture design, correct usage of caching strategies and requests optimization...
My Bottom Line
If you have some time to spend for learning new concepts (Groovy, RAD...), go with Grails. If not, go with Roo. Forget about Spring App and Rails.
If hosting is THE issue, so go with Python/Django. You can deploy on GAE, it's free scalable, performant and you will deal with the same concepts as Rails or Grails.