We've tried various deployments over the years with various levels of success.
Our 1.9 deployment is Nginx -> HaProxy -> Thin
I had trouble getting Mongrel to work in 1.9 (it was my favorite in 1.8) although apparently you can do it. Thin just worked(tm) so I switched to it painlessly.
I ran Passenger for a while in the past but I really didn't like how it would pause every rails process briefly when spinning up a new instance. Maybe that's been fixed now.
Monit may be harder to set up, but it works very well once it's running.
Ruby has a memory issue with long running processes. They seem to grow and grow and need to be restarted when they get out of hand. People commonly use Monit or God for this.
When you use Passenger, you can have it restart your processes after a certain amount of requests to prevent the memory leak issue, but when it happens it seems to block all requests to all processes for a short time, which is annoying to end users. It means your site can be speedy and zip along, then just pause.
Our 1.9 deployment is Nginx -> HaProxy -> Thin
I had trouble getting Mongrel to work in 1.9 (it was my favorite in 1.8) although apparently you can do it. Thin just worked(tm) so I switched to it painlessly.