Don't release until it's finished!!!
I've been waiting just as long as anybody for this mod, but with the cancellation of STALKER 2 I feel like a complete mod would be a more positive thing for everyone. Rushing out a beta is only going to bring criticisms, questions and bug reports that you are probably fully aware of already. Quality work in STALKER mods is hard to come by. Pretty much every one I've played has some incompleteness to it (including the main game itself!!). It would be a real change of pace to see a 100 percent complete and relatively bug-free mod released, and for you coders it would also reduce the time spent fixing bugs or incomplete bits later on.
I also worry that if you *do* release early, you might not feel like finishing it anymore. Where would the incentive come from? The surprise is ruined, some of the people will grouse and you'll smack your forehead and walk away. (I'm not saying that it will happen, only that I'm worried that it *could* happen).
The original game was released before it was finished, with negative consequences, and people have waited for years now to see a game that lives up to the original promise. I can wait another few years if you guys can make it happen!!!
You know what would be great, is a status page like DOSBox has, with progress bars and percentages of how complete each part is. People can look at it at a glance from time to time and know the status of the project's individual parts. It might even help you to recruit more volunteers to help out. Look here:
http://www.dosbox.com/status.php?show_status=1