What is different here is that I do a little development on one platform, then on the other - but being sure to use both RoR and JSPs+JavaBeans for new coding (vs. simply porting what I have already done).
I am finding Ruby on Rails to be a faster prototype/development environment, mostly because it takes over 10 seconds to re-test under Tomcat (remote access via IntelliJ) and re-testing under RoR is almost instantaneous. Ruby also is a little faster to code in.
No way can I put an accurate number on how much faster Ruby on Rails development is, but I would estimate that I save about 20% of development time - not a huge deal, but nice. This comparison is actually comforting from a Java perspective: for customers who prefer Java on the server, the extra overhead is not too bad.
There have been some comments that Ruby and RoR development is more "fun". I don't know about that: I really enjoy writing Java web applications with just JSPs, JavaBeans, and perhaps a few custom tag libraries. I must admit though that the "fun level" decreases when I have to mix in JMS, EJBs, etc., but that is just because I can't code from memory and have to occasionally stop to look stuff up.
Anyway, now that I am adding classes and behavior for groups, access control lists, etc., I am droping the Java half of this experiment. This is not saying anything against Java: I just want to beef up my Ruby on Rails experience and now get something done for my own use.
PS. a little off topic: I always think that I am a little bit more productive when using Linux or OS X than when I develop under Windows XP. This is a small effect though, and just a feeling, really.