Ive use TS2/TS3/Vent/Mumble, and my personal favorite is mumble by far. With TS3 coming in second. Vent is just bad, I hate how popular it is. And TS2 is absolute crap (even worse than vent).
The feature that keeps my on Mumble is how low you can set the latency on the CELT codec. TS3 can use the CELT codec as well, but to my knowledge you cannot adjust the buffer size on it. When you are comparing just audio performance Mumble wins hands down, no contest. TS3 could catch up if they allowed changing the buffer sizes on the audio packets. (Please note I have actually never run a TS3 server so I don't know 100% if you can change or not, if you can change it to match Mumble, I might actually like TS3 more)
Now the GUI can go either way. Personally I hate how cluttered TS3 is compared to Mumble/Vent, but some people may like having ranks on their server. Personally to me I don't give a shit if someone is titled "Admiral Captain Server Admin Rank 54545" I just want to talk to the guy. Maybe some clans might like this, but I feel its unnecessary on a VOIP client.
The settings menu on the other hand is terrible in my opinion. Its just set up terribly, here have a look:
I'm not saying Mumbles is perfect, but its a hell of a lot cleaner. I think they need to reorganize the tabs to this:
General
GUI
Audio Input
Audio Output
Notifications
Shortcuts/Hotkeys
I know they don't want to hear this, but they need an overhaul on the GUI, its just confusing as shit to find stuff. I don't need to see what operating system someone is using, if I want to know ill just ask them. The whole thing is just a mess IMHO.
I absolutely love the FTP feature of TS3!!! Its really really really really amazing. Definitely makes TS3 stand out from the others IMO.
I know it sounds like I'm bashing the crap out of TS3, but understand I think it has potential, which is why I'm doing this. I know Mumble will never become popular because it doesn't have all those unnecessary flashy lights the other VOIP programs have, but if they could just clean up the UI and allow dynamic codec buffering I would imagine it would be the best VOIP client.