When you choose to work in system administration, generally you have to focus on one operating system or another. The dichotomy comes in choosing a system to focus on for your career.
How do you go about choosing which system you want to administrate as a career? Do you go with a common system like Microsoft Windows or a relative rarity such as OpenVMS?
If you go with Microsoft Windows Server, for example, there will always be jobs available (relatively so, anyway). Every corporation seems to have at least one Microsoft Windows Server, and they all need to be taken care of by someone who knows what to do. However, there will be lots of other people that do the same thing. So even as there are jobs out there, there are lots of applicants and lots of competition. With this abundance of people who know how to administrate Windows servers (or think they do) comes a lower pay, as an employer can be selective in who they choose. This is the basic economic principle of supply and demand at work.
On the other side is administering UNIX servers – or even more so, OpenVMS servers. The number of people who can administrate these servers is less than those who work with Windows, which means their expertise is more expensive. For a variety of reasons, UNIX is present less in the average enterprise, and the number of UNIX servers is very likely dwarfed by the number of Windows servers. This is an advantage as the pay scale will be higher, but the disadvantage is that the jobs will be fewer.
When the market is tight, those with more specialized skills will find themselves having to move where the work is, and will have to search further afield for possible openings. It is a trade-off – and it’s your choice. Just be sure you have the facts first before you choose.