Having worked at a nonprofit startup, I agree from the perspective of an employee.
Since we were nonprofit, it was easier to justify being open source -- which meant anyone could contribute. Imagine being able to fix that pet bug or feature on your favorite site that the developers just don't have time for.
Same perks, salary, types of coworkers, etc. However, you also get the feeling that you're working for a greater good and not just bottom line of your investors.