should i start learning linux (to find work)?

Don't think of it as one or the other side

Microsoft has their own Linux department, after all, for making their tools compatible with mixed environments

Linux and Windows Server each have different reasons to choose them in the pre-development stage of a program or system. The biggest reason that a company would not choose Linux these days is from lack of available talent, however the available talent pool for Linux developers is generally more skilled than the talent pool for Microsoft platforms.

The old Linux vs Microsoft war used to be purely in the heads of management types that had no technical knowledge, but today those same types are becoming more trusting of open source technologies, as they become proven. There are still many companies who do not allow development on open source platforms, for example Health Canada does not allow PHP development. The reasons for this are in lack of information - that the management that makes that decision does not have the information to properly support a decision, and Microsoft does a good job of releasing white papers and marketing material about why their platforms are the proper tool.

In my experience, Linux is better for self contained systems. Ie, if you're starting from scratch on what you intend to be a large platform (or small) and you know it will not need to deeply integrate with other systems, then Linux can perform that task superbly.

Microsoft excels at integration between all of their products, but in my opinion this integration results in less CPU cycles being dedicated to a programmer's code - ie makes the system slower. This is due to the extra code Microsoft builds in to to cross checks between it's systems.

Both are great platforms, but it is good to have a diversified resume as nobody is truly considered a "Microsoft Platform Development Specialist" - if somebody only has Microsoft platform development on their resume, then instead their experience is considered to be limited. Linux is the opposite - somebody with only Linux experience on their resume would likely be assumed to be a Linux expert.