Is one a subset of the other? Do they have the same features? I may be running low on google-fu, but I can't find a feature comparison chart or anything.
Wikipedia > Google when looking for straightforward info ;-).
Aptana Studio supports Ruby on Rails development using RadRails, an open source plugin for the Ruby on Rails framework.
In essence, RadRails is an Eclipse plugin, and Aptana Studio is a commercial Eclipse-derivative. That includes Radrails in the package.