Teamspeak’s voice quality is higher than Ventrilo when the admin’s set the server up correctly.
Ventrilo uses GSM 6 and 11. Teamspeak uses CLEP 5 and 7, GSM 14 and 16, and Speex 3, 5, 7, 9, 12, 16, 20, and 25. Each of those represent Kbit/sec baudrate.
If Ventrilo was still free I’d say use it but it can’t do anything that Teamspeak can’t.