I would say both types have the same effect for me. I like knowing that I made even a small difference in someone's day/life.

I used to volunteer in the Emergency Room at a local hospital and also as a volunteer EMT. It can be pretty depressing seeing some of the cases that you come across, but a surprising amount of people, when faced with the option, still prefer to stay alive even when they are in so much pain and discomfort. Not saying that that is a reason, but it is a fact.

The informal types are a lot more satisfying to be honest, if only because you don't have to deal with the rest of the hassles of working through a job or organization.