We used Phusion Passenger, although we have plans to add additional servers (such as Unicorn). We tried to spend time with various server choices for all platforms, and for ruby, in our short test, Passenger won out against the others.
Our understanding is that when running Passenger, simply passing '-e production' to the command line is sufficient to run in production, but if that's incorrect, we'll gladly update the test.
Please make sure you're setting higher GC limits for the Ruby tests. Ruby's defaults are awful for a framework, and result in a LOT of GC thrash. It's not uncommon to see an order of magnitude improvement in performance when they're tuned properly. (edit: I'll just send a pull request, I found the setup file!)
Something else you might consider is the OJ gem rather than just the stock Ruby json gem. The latter is notoriously slow and memory-hungry (which will compound the GC issues!)
For anyone still lurking, this user replied to me via email:
Ruby allocates heaps for its objects, and sets GC thresholds based on those heap sizes. Ruby allows you to change those settings via environment variables, which means that you can end up doing fewer allocations and less aggressive GC, which makes sense when using a full framework like Rails, which is going to allocate a lot of objects.
Our understanding is that when running Passenger, simply passing '-e production' to the command line is sufficient to run in production, but if that's incorrect, we'll gladly update the test.