Now that's interesting. I use both every day. I find Java to be perfect for UI work, but for me nothing beats the power of RPG for database access. And with the new releases of RPG, you can create external procedures that have many characteristics of Java methods. It's not exactly OO, but it makes for some really powerful programming. Add all the built-in APIs for stream file access and sockets and all that and it's a pretty powerful programming environment. For example, I wrote a basic XML parser with callbacks in about 100 lines of code. But primarily, my architecture is servlets invoking RPG business logic to fill beans to send to JSPs. Classic JSP Model II stuff.
Ah, Map would disagree with you