The operating system OSX is seen as more stable and protected against viruses than Windows. That is the reason why most companies and universities like to buy Macs instead of PCs. Also, most universities have a discount on Macs. Granted, for private use and gaming the PC is way better.
Linux is vastly superior to both. It has better security than both (with most software for the platform being open source), far more customization options (which contribute to a more efficient workflow), and it's completely free to use, copy, modify, and redistribute under the GPL license.