Being a Novell CNE for many years, I took a Microsoft exam on Windows 2000 Server, and earned an MCP. I am currently working on Solaris certification. If you master the general concepts of any of these systems, you can get a good handle on the others. Of course, specific commands and how you do things are different, but the concepts are the same. I'm going with Unix because it's been around for a long time, and it's stable. Also, if you learn Unix, you can figure out lots of other things - like Cisco, Linux, AIX, etc.
One tip that I'd like to pass along is this - The Novell exams and the Solaris exams really test your knowledge and experience. I felt the Microsoft exam tries to "trick" people into selecting the wrong answer. If I were hiring, (and I'm about to) I'd much rather have someone who learned how to use the product, instead of learned how not to be tricked on an exam. So, if I were you, yes, I'd get Unix certification, especially if, like me, you really prefer Unix.
Iolair MacWalter