I would prefer TeamSpeak, but Ventrilo is good to. TeamSpeak is more friendly user though.

Some are saying the voice quality on TeamSpeak is not so great. I disagree, not only does the quality depend on what the user's settings on TeamSpeak are set to, but what codec is set for that channel.

It's really a matter of personal preference.

Quote from another site:
Well then, again, my original point stands. I don't believe there's going to be any appreciable difference between the two; it will boil down to personal preference. In today's world of 2+ GHz computers and 1Mbps + broadband connections... you probably aren't going to notice a performance difference between the two.