Try both NetBeans and RadRails for maybe a week each, then you can find which works best for you. The best advice is to learn your tool. If you are not checking out something new about your editor, something that could potentially save you time (regexp, etc) then you are doing yourself a huge disservice.
I have been using Eclipse/Aptana/RadRails and unlike Gaius have been pretty happy with it.
I recommend the Eclipse IDE for Java Developers from Eclipse Downloads: http://www.eclipse.org/downloads/
Then grab Aptana Studio, following these instructions.
When Eclipse restarts Aptana will have a view, click on rad rails and you are good to go. Just make sure you have ruby installed already, or it becomes a pain to resolve.