In this page you could hear some codec samples to compare the quality. When the sampling rate is higher the quality is worse. On the other hand, the sampling rate reduces the bandwidth necessary to send the information and therefore the delays.

5 seconds sample:

[.WAV] G.711 - 64 kbps
[.WAV] Intel/DVI ADPCM codec - 32 Kbps (very similar to G.721)
[.WAV] GSM codec - 13.2 Kbps
[.WAV] LPC10 codec - 2.4 Kbps

25 seconds sample:

[.WAV] G.711 - 64 kbps
[.WAV] Intel/DVI ADPCM codec - 32 Kbps (very similar to G.721)
[.WAV] GSM codec - 13.2 Kbps
[.WAV] LPC10 codec - 2.4 Kbps
.

Thanks to Hawk Software project. You could find the information in http://www.hawksoft.com/hawkvoice/codecs.shtml