I've used TS2, TS3 and Vent through the ages, I've used it for EVE, COD 2, 3, 4 for clan warfares, World in Conflict ESL and CoH and been an admin etc on both TS2 and Vent.
Here are my thoughts.. (ALL opinions of course)
I found TS2 the most difficult, Vent is easy to use and simpler.
Because of this though TS2 was much more powerful if not a bit unwieldy.
TS3 has improved the interface vastly over the old TS2.
TS3 has excellent audio quality and the option of Codec's is vast. However choose wisely as some of them will sap your net connection dry...
Vent voice quality was generally poorer even over TS2 but the delay tended to be less, possibly down to lower cpu cycle demands on the lower compression, or maybe due to low latency never really looked to in depth as to why.
TS3 provides the easy use features of Vent, but the power of TS2, it still has it's quirks, but generally it's much more streamline and doesn't have that annoying TS2 username glitch.
At the end of the day Team speak I feel is a lot more heavy duty than Ventrillo, Vent used to start to hiccup at around 150 members, later itterations have improved this dramatically it is still much more comfortable at the corp level rather than at alliance or even coalition size comms, but even in the old days, Teamspeak could handle 3 - 4 hundred without to much problem. Unsure if this feature is to do with servers we used or the program itself.