I like teamspeak a little more because it's been around a lot longer. I acknowledge that Ventrilo is higher quality and more reliable than teamspeak but teamspeak had a very nice channel/server system, it looked a lot nicer and it got the job done just fine.
Also I found a few very nice overlay programs for it. I haven't used it for a FPS ever though.
Ventrilo is kind of the standard so I don't mind using it.
I've used Ventrilo over half of my FPS gaming 'career', and i can say the voice quality has been superior to most other programs until TeamSpeak 3. TeamSpeak 3's voice quality is almost up to par with Ventrilo, it's also as simple to use. I actually use both daily. Vent for eGO and TS3 to chat with a few mates.
Teamspeak 3 is the best teamspeak to come out by far. the voice quality is 99% that of ventrillo, but it works better for people with slower connections because it requires less data to be transmitted. also the channel setup is much easier in teamspeak and each channel can be customized a lot easier. most people will vote for vent because they are more likely to have used it for FPS clans and communities. however, i vote teamspeak because it is much better all around: easy to use and requires much less bandwith than vent.
Actually, Teamspeak doesn't require a license so it would be cheaper, that way we could host it on one of our boxes instead of paying more at a retailer.
Anyway, after using teamspeak, and seeing the features it has to offer admin wise, as well as client side, I vote Teamspeak. However it would take some getting used to.