Give time or give money?

In the October 14, 2006 edition of Slate magazine, Tim Harford writes in his article, Charity is Selfish: The Economic Case Against Philanthropy:

"If these do-gooders really were motivated by the desire to do good, they would be doing something different. It would almost always be more effective to volunteer less, work overtime, and give more. A Dutch banker can pay for a lot of soup-kitchen chefs and servers with a couple of hours' worth of his salary, but that wouldn't provide the same feel-good buzz as ladling out stew himself, would it?"

Is it better, or more effecitve, to give time or to give money? Should that question matter - is it really about what will help the recipients most, or what will make you feel better - or some combination thereof? And who said that charity has to be selfless? I think of charity not as an option, but as an obligation, so it's really not a selfless act for me. What do you think?