Both NetBeans and Aptana (Aptana has taken over RadRails) are useful, and both have their drawbacks. I usually have both open at the same time, editing the same files.
Aptana/RadRails
- Subversion support is much better than NetBeans.
- The test runner system is very useful, and NetBeans doesn't have anything like it.
NetBeans:
- subversion support on windows is poor (they depend on using external tools, and they don't work with cygwin svn)
- better code completion
- much better debugging
- the NetBeans team responds to bug reports very quickly
I use NetBeans for the debugger, and Aptana when I just want debug printfs.
I don't really have much hope for anyone's code completion in Ruby for a while. Too many things are dynamic; the chances of doing full code completion without a running application are close to zero. Seems like you're going to need very tight integration between the IDE and a running process to make this work.
You want to be running the nightly (hourly, sometimes) builds for NetBeans. Today, you'd want netbeans-rubyide-hudson-2749.zip from http://deadlock.netbeans.org/hudson/job/ruby/
http://wiki.netbeans.org/wiki/view/Ruby