Pretty sure most of the developers are using Macs - not 100% sure, but
that’s certainly the impression I’ve had to date.
As far as FOSS goes, although I personally would recommend Gentoo or
Debian Linux, I’d go with whatever you’ve got the most sysadmin
expertise in. If you’ve got a FreeBSD sysadmin guy who knows his
stuff, use FreeBSD. There’s lots of Rails production systems running
on Macs. If it’s Windows, umm, well, I guess you could do that also,
but you want FOSS so I guess you’re probably choosing between *BSD and
various flavours of Linux.
Really, I don’t think there’s much to choose between any of the OS
variants as a Rails hosting platform; I like Gentoo because I’m
familiar with it and it consumes the least amount of my time per box
to keep it secure and current. Other people like Debian for pretty
much the same reason - they’re more expert than me with Debian, and
I’m possibly more expert with Gentoo.
Have you got a compelling reason to move away from Ubuntu? You’ve
found a few challenges with it, but you’ve presumably worked around
those problems to some extent and already invested something in it to
date as a Rails platform, so I’d put it at the top of the list unless
you’re already more expert with something else. All platforms have
limitations; at least you know what they are with Ubuntu already.