I've found that Ventrilo has higher sound quality for lower bandwidth usage.
To put it in other words, to match the average sound quality of Ventrilo, TeamSpeak would have to be using a low-compression codec that would use more network traffic than Ventrilo would be.
Also, Ventrilo has a better interface for keybinds and account setups.
One thing I have noticed, though, on machines with integrated sound devices, heavy processor/video rendering will activate the microphone in Ventrilo due to internal 'noise'. TeamSpeak seems to handle that part a bit better.
I'd go with Ventrilo.