arne,
I have loaded Ventrilo onto my server (Server only mode) 6.01-01 custom and it worked fine. The only issue was setting up the client on the windows end. I was trying to use the smallest amount of bandwith as possible but were having some issues with the client end.
Server worked fine, but some minor config issues with the client.
The difference from my reading is that Teamspeak will allow as many users as you like for free, where as Ventrilo will only allow 8. Also the TSS tells the client what codec to use, whereas Ventrilo you need to configure it. Also the bandwidth used by TSS is higher than Ventrilo.
If you want me info on how I install Ventrilo please email me and I will see if I can find my how to I wrote at home.