If you can write off the donations (for some people the amount they'd get for itemizing their deductions from donations won't be over the standard deduction so they'd be better off just taking the standard), then I think it depends on whether you want the money now or can hold off until tax season. It may not be as much by selling at a garage sale, but then the money is immediate. The last sale I had was a disaster. The neighborhood was having their neighborhood sales so we took a part of that. But the weekend that was chosen was cold and rainy all day (just hovering in the low 40s with a constant drizzle), so no one was out buying. Also since I'm living alone (the "we" above is that a friend also put up some of her stuff for sale too) I had to work all day weekdays; I was able to take that Friday off for the sale, but unlike other neighbors I wasn't able to have a sale on that Thursday too. I think after accounting for all of the supplies for tagging the items and making signs for the roads, I took in only about $20.
I also think garage sales are good if you have kids (at least kids that can be entertained for a few hours). I went to one and the kids were busy selling their toys. They were learning how to be a salesperson, how to handle money and give correct change, they got to do some craft work by making signs and other decorations, they learned how to bake (they had cookies for sale as well), and in general learned how to give away their possessions that they no longer used.
I like going to garage sales, esp. for CDs and books, but it sometimes is a waste of a day... Some weekends I'll spend the whole morning on Saturday driving around from garage sale to garage sale coming across all sorts of baby clothes (too bad I'm childless), but nothing for me. Then again, even if I don't buy anything, it's nice to be able to drive around listening to my favorite CD and just enjoying the nice weather.