I usually code in c# so figuring out which editor to use is pretty much a no-brainer. Well with Ruby there is a little bit more of a choice. When I first started looking into Ruby I had been using the RDT plugin for Eclipse which was great but I noticed recently that they haven't been doing much with it. This doesn't mean it's bad or anything, I mean it still works like it did before, I just thought that maybe there were bigger and better things now-days. So my quest began. I remembered RadRails but wanted something that was good for plain Ruby development and Rails development. RadRails was basically for Rails. I looked into just using Emacs, but I wanted something that could debug and do all the fancy things I was used to. There was ActiveState's Komodo IDE which looked really great until I saw that there was a price tag, and a hefty one too for someone like me who was just tinkering. Then I finally stumbled accross TextMate and Aptana's RadRails. These are both really good (TextMate is only really good if you have a Mac though, no Windows/*nix versions). I noticed that everyone on the Rails team uses TextMate, but I've used Eclipse before for Java dev and really liked it so I was kinda stuck.
So I guess the biggest thing for me right now is the price. TextMate is great and everything.. but it's 39 Euros (About $53 US) and Aptana's RadRails is free and is completely focused on Ruby only whereas TextMate is a text editor for lots of things.