I switched from Aptana to RubyMine earlier in 2012 and have not looked back. I find that RubyMine excels at code insight, code navigation (jumping between models, controllers, etc.), refactoring, database view, version control tools, and that’s just my highlighted features … lots of other goodies packed in there.
One nice bonus with RubyMine is that it supports Sinatra as well as Rails, which is nice if you end up
The one feature that I like better with Aptana over RubyMine is that Aptana has the terminal built in. That’s pretty nice but, ultimately, it was not a big enough deal to keep me on Aptana. Additionally, Aptana’s progress has felt stalled since Appcelerator bought them up … most of their development efforts seem focused on Titanium Studio, which is Aptana rebranded and enhanced for mobile application development.
For a lightweight “IDE”, Sublime Text 2 is excellent. I prefer a truer IDE but that’s just probably because I am from a Java/ColdFusion background and that was the most common approach to development (use of a true IDE versus how Rails devs were brought up with TextMate back in the day).
Ultimately, try both for a small project each (or just a tutorial project) and see which one fits you best because, if you are comfortable in it, enjoy using it and more productive with it, then that IDE will be the right one for you!