With my experience, TS2 has the easier setup and troubleshooting. Vent uses some of the controls established by Windows or the OS in general; while it continues to administer its own settings for other things. This can make initial setup complicated. There are more features generally in Vent, but many are useless like the ability to see what someone is listening to on their audio player.
TS2 Grabs all its settings except for the voice activation levels from windows, this makes it so if someone has an issue the source is easy to find and simple to troubleshoot. the Keybindings are useless and really do allow someone a bit of flexibility while working with the app. Teamspeak 3 when it is released will add many of the features Vent has as an advantage over TS2 so keep looking to the future for that.
There is another VOIP app coming out as well called Mohawk, After a brief experience checking it out it adopts many of the features from Vent but tries to simplify them like TS2. They're intending to add some of the features included in Xfire as well. Generally the sound quality was reasonable, and the process of getting it all setup and calibrated was somewhat painless. The UI functionality was horrid, and the admin controls were miserable. After talking with the lead dev and tossing a ton of suggestions those were supposed to be worked on. That was some months ago so I haven't bothered to check up on it since.
Haven't heard of Mumble so no comments on that one, will check it out in the future.
All in all, I will stick with TS2 most likely until TS3 comes out, then I'll make the simple upgrade and get the ball rolling on getting everyone setup and functioning.