I've been using Phusion Passenger + Rails/Sinatra for a lot of projects. Passenger runs under the main Nginx or Apache process.
But I'm interested in Unicorn, partly because it runs in user space. You just set up Nginx to proxy_pass requests to a unix socket that is connected to Unicorn processes that you fire up under a normal user account.
Is there anything to be said as far as advantages and disadvantages of these two alternative approaches to running an web app? I mean in terms of ease of administration, stability, simplicity, etc.