There are a lot of factors that go into call quality. Overall, 3G is supposed to have better quality than 2G, but depending on location and some conditions, 2G can be just as good.
For example, when making a GSM (2G) call, there are several different codecs that can be used to make the call. AT&T uses a half rate bit codec in heavily populated areas and during times of peak usage to be able to squeeze more calls in. Half rate codecs typically sound a bit more "digital" and are more prone to garble. However, if you are lucky enough to connect the call using a full rate codec, then 2G and 3G are pretty much identical. The only difference would be during hand offs, where 2G uses "hard" handoffs during tower transitions which most users hear as garble or the call cutting out for a half second. 3G does not have that problem as it uses "soft" handoffs.