linux is NOT a offshoot of unix.. and omg, Minix is in design before unix is ever conceived.. unix is produced because they think minix too complex (read some more history before posting please?)
linux is designed by linus to be "like" unix, whatever that meant, it based on no unix code.. though, current kernel has alot of freebsd copyright notices..
freebsd is based on berkeley unix.. so it's a true unix, and linux is just trying to imitate unix.. a good way to think about it is, bsd is made by unix hacker to be unix, and linux is made by windows hacker to be like unix.
there are several fundamental differences, freebsd community is violently hostile to new distros, as they think it distracts from the main branch.. there is a new distro of freebsd that's supposedly more userfriendly, PCBsd, but if you search on bsdforums.org, you'll see the typical attitudes of freebsd community
also, there are differences in naming conventions, and various other stuff different enough to make your heads spin... it's used mainly for servers, and because there is so few distros, each one is very very stable, more so than linux..
it supports less hardware, but most hardware can be made to work if tinkered enough, with some luck, you'll make freebsd work just fine
it also runs 80% of linux binary without much problems.. (it runs because it has linux compatibility layer on freebsd, it'll as u if you want to install it at the beginning.. please, you should really check some of your facts :|)
freebsd is the closest you'll get to a modern unix