I'm afraid that I haven't worked with Oz, so can't comment too much. I've done some workon Erlang, however, and would have to rate it as a good choice for certain types of application (it's great for state machines and protocol stacks - no great surprise given where it came from).
Erlang does have the advantage that it is a reasonably mature system - there are industrial scale telecomms and networking systems (mostly embedded) out there which use Erlang, and work very well.
It's worth remembering that Erlang grew out of Ericsson's need to try to streamline its won development and software QA issues, and so addresses particularly successfully some of the main preoccupations in telecomms networking: very high availability and reload of software on the fly.
On a personal note, I find that Erlang appeals to the same part of my brain which loves working in Python, although it is, of course, very different in look and feel...