If his (her?) budget was lower, I'd definitely recommend a garage sale bike. The trouble is that you don't really know what you're going to get with them quality wise unless you already know what to look for. As long as the used bike store is reputable, the quality there should be more consistent, and if things do go wrong they would be more likely to help him out for free/cheaper if he bought from them. Besides, it's always a good idea to have a positive relationship with the LBS, and having a variety of bikes to try out might be helpful. On the other hand, if you're willing to put in some effort learning maintenance and repair, a garage sale bike can provide a good ride for cheap.