I often thought previously that voice chat in the game would be nice. But I was just thinking, the main reason to have voice chat would be to talk with teammates about strategy, etc. To have it in the game, everyone would be in the same chat, thus cancelling out the benefit of team voice chat. That is why you have Ventrillo and Teamspeak other than simple chatting with your buds.
About the TS versus Vent. I can't use either because I'm on dialup. I have attempted to use both. The difference is with Ventrilo, as soon as someone would talk, I would lag out and loose connection. In Teamspeak, I would only lag on the server, so I'd say that Ventrilo has the heavier bandwidth usage. Quality wise, I didn't notice a difference between either of them. I do use TS with my online flying when we do ATC sessions. But that is a completely different way the game to server connection is. I can download stuff, surf in a browser, us TS, and fly online at the same time with my dialup connection. I can only race with no other stuff running across the net connection with LFS or it's a server disconnect.