You're working against the grain if you go BSD instead of Linux with Rails, whether you like FreeBSD or not.
For the past several years, and up until FreeBSD 7 you've always been seriously misled. FreeBSD has historically had dramatically worse performance in most concurrent server applications (certainly in multi-process SMP workloads). As of FreeBSD 7, it generally is the equal or slight better of Linux. It is not better in enough categories or by a large enough margin for anyone to say with confidence that a particular workload will perform better on FreeBSD or Linux. One would always have to benchmark the actual application, and the differences would be minimal in most cases...and with every new kernel release, you'd have to benchmark again, because things change and performance advantages might flip from one release to the next.
And, tptacek is correct, none of the other BSDs has inherited these dramatic performance improvements found in FreeBSD 7, so they are very unlikely to provide better performance than any modern Linux version on modern multi-core hardware.
I personally use Debian for everything. I can't imagine the distro making a huge difference really. I've used Debian as a desktop os for some time, so when I needed a server os, it was easy to get going quickly.
Switching from Apache to Nginx made more of a difference in my case than anything else I could have done I believe.