Greenhouse isn't ready to be compared to Atlas just yet, the development has only been ongoing for a matter of weeks. That being said, Atlas costs money, and Greenhouse is completely open source:

http://github.com/sproutit/sproutcore/tree/mg/master

If you're more comfortable with Obj-C/J, then Cappuccino is the way you should go. Personally I use SproutCore because I'm far more comfortable in JS/HTML/CSS.

It doesn't do much. Atlas works.

Greenhouse: Open Source.

Atlas: Costs Money.