I play WoW and have used both. For serious raiding, I lean towards Vent for the reason "Anon" above mentioned. But for PuGs I use the in-game option. I don't like it, I can NEVER get the volumes right (I have to turn the in-game sounds WAAAAAAAAY down and leave the voice up all the way, and then crank my PC speakers to make things audible, and then any time anything outside of the game happens, my ears bleed XD), but I'd rather deal with that than giving out private Vent server info.