I must admit I had rather limited experience with mumble and I've used vent rather long time ago, but from what I remember, sound quality is pretty much the same across all of them, but it seem that mumble has a better one by default, so if you're not tech savvy, you might wana stick to that.
What annoyed me the most in both mumble and vent is the amount of setting up I had to do when first installed. In vent I had to change half of the sounds and most of the sound level setting to sound properly and not get annoyed by the sound effects it had. Regarding mumble... well, I barely remember it, but I clearly remember being angry at it for some reason when configuring. This is why I like TS, since all I need to do is change it to voice activation and adjust the activation level.
Vent also always annoyed me with it's interface, it was just too small and clumped up. And mumble's interface was rather dark from what I remember, at least the default one. Can't really comment on TS here, since I'm so used to it it will just be biased.
But I've heard some nice things about Mumble, mainly that it has lower latency (tho we still never had problems with supposedly higher one on TS) and ability to buy server for specific amount of people, which is really nice if you know how many people will be there exactly, but we always proffered to have some extra slots (right now we lease 10 slots to a friendly guild, so it actually came in handy).