Unix is older. And its open source so its kept up with changes right up to today. But Unix is a stuffy old crow. It concentrates more on power, security and stability. It has much fewer new toys. 

Linux is prettier, many more toys, quicker to get into the new fun things being done on the net. But for that reason, it tends to be plugging holes alot more.

When an internet server company says they want to create a webserver or email server or news server.. and they want it large, secure, and stable so they can pretty much ignore it once its running... then I recomend unix to them.

unix is also good but maybe not quite as flexable I have PC-BSD on my laptop just cause I couldnt put on a linux OS