Some topics of discussion tend to get argumentative, and the Mac OS vs Windows is one of the most debated out there (of course,
next to Android vs iOS (Android vs iOS : Which do you prefer and why?) 
). On one side, we have Mac OS, where the manufacturer has complete control over the hardware & software, while on the other hand we have Windows, which is a software that runs on machines made by other manufacturers. This thread is to discuss the merits and demerits of each, and help others take an informed decision on what to pick.
Mac Pros & Cons:
• More expensive to buy, although I will say Macs last longer. As an example, I'm typing this post from a 9 year old iMac. I don't know of too many 2011 Windows laptops still running. At least not smoothly.
• Tighter control over the hardware and software ensures a seamless experience for the end user. Most people will agree that MacOS provides a cleaner, purer experience.
• Less software support. If you get a Macbook, odds are that some of your favourite software don't run on it.
• A spec-war might not show a Mac as the leading choice for a given budget, it's the overall functionality that matters. A Mac requires lower system resources to deliver the same output.
• Top class display quality.
• The newer Macbooks sadly don't offer any possibility of user upgrades. E.g. the RAM is soldered on.
• Further to the previous point, customisation options (hardware) are almost zero.
• Bundled free apps include an office suite, but you will still need MS Office.
• Out-of-warranty repairs are eye-wateringly expensive.
• Even with the extended warranty option, the max you can get coverage for is 3 years. In comparison, many Windows PCs offer you 5 year warranties.
• iOS and Mac OS integration allows you to seamlessly continue your work from one device to another, if you are an iPhone / iPad user.
• Usually, less issues with drivers, blue screens etc. Even the software updates are hassle-free.
• Macs "sleep" better and "wake up" without a fuss.
• Certain creative apps are Mac only, especially from the Audio / Video / Music industries.
• Superior security and higher stability due to the control over hardware + software.
• Your choices are severely limited in the Mac OS world. As an example, if you wanted a laptop, there is just the Air, 13" Pro & 16" Pro. With Windows, you probably have 500 options.
• You can run both (Mac OS & Windows) on your Mac, if you so wish, thanks to BootCamp.
• Apple's computers & laptops (especially) enjoy good resale value. You can still get a healthy price for your 4-year old Macbook. Try that with your Dell or HP.
• When it comes to gaming, Macs are at a disadvantage. Most advanced gamers consider Windows for their choice of rig.
• Last but not the least, Macs are really, really beautiful computers. They look fabulous.
Windows PC Pros & Cons:
• An almost unlimited choice of brands, and something for the budget conscious as well as loaded customers.
• Going by Microsoft's market-share, odds are that 80 - 90% of BHPians are already familiar with Windows. There is no learning curve if you buy a new Win computer.
• With very few exceptions, you won't get as good company support for your Windows laptop as you would for an Apple computer.
• You could buy two premium Windows computers for the price of 1 high-end Mac.
• Due to a larger market share, most of the apps / software and games first come to Windows. Many times, they don't go to Mac OS at all.
• Many machines come with touchscreens, pen inputs, etc. catering to a wider audience.
• Hardware choices are innumerable and users can pick and choose the exact components they need.
• Hardware can usually be upgraded from time to time across the ownership period.
• Superior VR and 3D support.
• Windows is the preferred choice for gamers, offering a larger pool of games than Mac OS.
• Sadly, Windows is more susceptible to viruses. You can't be using a Windows computer without an anti-virus running in the background.
• 5 year warranties possible from most of the big brands!
• Almost all new PCs come with bloatware from the factory. No such issues with Mac.
• You can still use old stuff like traditional USB ports!
• With few exceptions, Windows computers & laptops just don't last as long as Macs.
• PCs get slow with time & there are innumerable bugs, BSODs & driver issues.
• Some of the new offerings (especially the Dell XPS range & Lenovo X1) score big on aesthetics.
• Out-of-warranty repairs are usually reasonably priced, and you will receive ample support in just about any corner of the country.
• Windows desktops & laptops usually have dismal resale value, unlike Macs.
