Ventrilo pwns teamspeak. Before you say anything noobish, i've used both a lot. Ventrilo by default has better sound quality than TS with speex bitrate at max, Ventrilo is also maybe more user friendly, speech programs need to be small and unseen (start, connect and forget), not like the size of the screen forcing the user to see it all the time.
The only minus Ventrilo has, that TS doesn't: No recording. I'd really want to record clan wars n'stuff, but Ventrilo cannot do it.