I bought an ultra mega cheapo computer for just doing UNIX socket programming with Stevens' UNIX network programming book and some thread programming with another book by a different author. The programs in Stevens' book have been checked out on Linux and FreeBSD equipped machines. Anyone want to recommend one over the other? If so, why that choice?

try them both, dual boot if you have to.

I personally find FreeBSD is more mature and seems more stable(especially when it comes to sound drivers), but linux(especially arch) is easier to set up and keep up to date.

What's more important to you?

FreeBSD or OpenBSD are the way to go. You can grab an ISO for either that will boot and install from the Internet. The full installations are a lot smaller than most Linux distros and include everything you need for development. Debian Linux also makes for a good minimalist system.