For both the AAC and MP3 encoders, GarageBand offers four preset compression settings: Mono, Spoken, Musical, and Higher Quality.
Below is the information GarageBand displays when selecting the various MP3 settings. I've included the size of a sample podcast for comparison. Notice that a sound file in the Higher Quality Encoder setting is six times larger than the same file exported using Mono Encoder.
Shouldn't everyone choose the highest quality sound they can get? Well, the higher the quality the large the file size. Larger file sizes take longer to download and take up valuable megabytes on file servers, in iTunes, and on portable players. If you use a hosting service with limits on storage and bandwidth, larger episode files might use up your monthly allotment.
GarageBand's presets make a really handy reference. In fact, podcasters using Audacity and other recording programs could use these same settings when encoding audio. You'll have to manually input the settings after choosing Preferences > File Formats. Under MP3 Export Setup, change the Bit Rate to match GarageBand's preset. For example, set the Bit Rate to 64 for a Spoken podcast.
Whatever software you use to compress your podcast's audio, I suggest exporting the same sound file with different settings. Have a listen to each one and balance sound quality with file size when settling on a compression setting.