My experience is that Unix is on the decline, Linux & windows usage is slightly increasing.
At my current employment, the Digital & Sun servers are being replaced with Linux (RedHat). The UNIX workstations are being replaced with Windows.
Windows is being used because the company developes software for Windows, and the fact that Exceed on Demand is so good their is little need for Unix desktops anymore.
And to answer the original posters question.
Learn both, a lot of compaines use Unix/Linux for the perimeter security and ftp/webservers servers for external access. Internally they have Windows workstations, and Windows servers.
If you want a good job
Windows - Learn GP, software deployment, user management, designing a infrastructure, clustering etc
Linux - Learn nis/YP, clustering, webhosting etc
Also learn a lot about security (vpns, iislockdown, dmzs), its becoming a *very* important skill to employers.
The best way forward would be to become a complete guru about Linux and know the fundamentals of Windows.
I you haven't got a job in IT yet, then it isn't going to a easy ride even if you do have the skills
Also if you only want to get into IT for the $$$ then I really wouldn't bother.