I'm currently working towards my CCNA cert going through the Cisco Networking Acadamy. What I need to decide is what netowrking operating system to specialize in, I've looked at the 3 big ones.. Unix, Novell and Microsoft.

I have no idea what i should go for. I do plan on upgrading my cert in a few years to CCNP and possibly beyond, so I don't want to pick something that will tie me down to LANs. Anyone want to make a recommendation or give pros and cons of the OS that you tend to work with