What are you planning on doing? Both platforms have advantages. Personally I do a lot of AJAX and web front end stuff which is what Ruby on Rails was built for.
Java can be quicker than RoR in execution but app design and roll-out tends to be a lot faster with Ruby on Rails. Obviously you need to doa bit of reading on the subject. Id sugegst the rather excellent RoR screencasts as a starting point for RoR.