actually, the biggest difference in Linux and BSD(and prolly the one that made apple choose bsd) is the licensing. The funny thing about the GPL which Linux is released under is that it acts sort of like a virus. If you base software on code that is licened under the GPL, then your software must also be free. This would be bad for a company such as apple, because if Mac OS X was based on linux, and required that foundation to function, then acording to the terms of the GPL, it would alos have to be free...
MySQL has a simalr situation, they release the MySQL server under the GPL, but they also offer a very resonably priced corprate licens so that if you have a product that depends on MySQL to function, such as some forum applications, or other db backed software, you can purchace the corprate licens so that you are able to sell your software, otherwise, if MySQL were required for yoru software to operate, you would have to give it away...