Tell them the Mac OS makes you work more efficiently.
the Mac Operating system can be your "Killer App."
In which case, we may as well tell the OP to say "FreeBSD is the killer app", since OS X was built with it (the Cocoa API being one of the few differentiations).
But lots of more stable, multitasking OSes competed with Windows in the 1990s.
Guess which platform won? (Hint, it wasn't the best... and was pretty much the worst and I had been around the geek block at the time, right down to OS/2 and BeOS, but the OS that had won was the best
marketed.)
Anyone with a technical background WILL shoot your idea down without more evidence.
Because everybody knows that the OS does not make a hill of beans in this equation. It's just a layer.
It's the applications that run under the OS that make the difference.
The OS is not an app, nor would we want it to be an app... I wouldn't want it to be an app.
So add in those costs and prove Mac Office works better than MS Office (forgive me, as I run Windows Office in a Parallels session, because Mac Office has always been crap, in typical Microsoft fashion..)
And then there's the issue of Windows->OSX ports and how well they are maintained. Netbeans has been slower to get updated on OS X... Adobe's certainly not kept up with Flash development (and there could be a better alternative for Flash that won't become as fragmented as HTML5
will eventually be)...
In many ways I will put OS X above Windows, but I know enough that "the OS is a killer app" wouldn't be swallowed by anybody.
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Let's see...
1) Mac has had multitouch gesture support for a long while now. Pretty much any application you use on Mac can be navigated with multitouch gestures. When you have to view many PDF documents or large photos, or huge spreadsheets, being able to scroll in both dimensions (up, down, left, right) arbitrarily without having to grab the scroll bar is something you won't be able to do in Windows just yet.
2) Mac has built-in Contacts/Calendar/Reminders/Notes suite that allows you to organize your schedule, keep track of your life, and make sure your clients and business partners are always within just a few mouse clicks. Windows 8 comes with the equivalence of these, but importing your contacts, calendars, emails, reminders, notes, etc... into Windows is a pain in the rear.
3) Mac has FaceTime, which allows you to easily communicate with clients and business partners who have iPhones/iPads. Not everyone uses Skype.
4) Some of your clients/contacts use Mac computers, and you'd like to be compatible with their workflow. Mac can see contents copied to Windows-formatted USB drives (NTFS, FAT, or exFAT), but Windows can't see some USB drives formatted on a Mac (HFS+)
5) Mac has Automator built-in, which allows you to automate some mundane tasks like batch copying files and organizing folders, creating PDF documents from photos with just two clicks of a mouse, automatically print all of the documents inside a folder, automatically inspect calendar events and then print document on a date, etc... You can accomplish about the same thing on Windows but through many hoops like third-party applications, batch files, etc... and even then, it's not like you can just copy all of your Automator actions over to your Windows computer like that.
If you were a programmer/designer, then I'd have another 20 reasons for you...
1. I use OS X and don't use that feature now. Doesn't seem to be a big selling point.
2. Most corporations use Outlook.
3. An article I read said that companies aren't wasting tons of money for big bling screens for the fancy face-to-face nationwide meetings and such because it's not worth the money. It wasn't needed before, there is no need for it now - not for corporations, who are so worried about "cost" that they'll eventually starve in the end anyhow... facetime and the other glitzy bling is for the customer consumers that prattle on facebook all day. No sale.
4. That could be justified as a viable point.
5. Windows 7 has some not-dissimilar utilities integrated. Otherwise there are plug-ins... they're not hard to find, so is it worth the massive additional cost?
/devilsadvocate
As for the claim of "20 reasons" for developers and designers... What are those 20 reasons?
Premiere Pro is for Macs, and that's a good one, especially after Apple bungled it badly with Final Cut "Pro" X.
As is Photoshop. I am using CS6 and liking it immensely, though after the latest patch Adobe put out I had to re-enter my license key... but that can conceivably happen in Windows as well.
So, we're down to 18 reasons left, but I do Java development with the Netbeans IDE and they are slow to put out OS X updates.
Never mind getting the JDK (I couldn't get 1.6 for Mountain Lion...)
Objective-C is older than the antiquated Flash platform and also lacks garbage collection, so that can't be a reason - it's proprietary to Apple and companies will scram from walled gardens the moment they realize the cost and benefits no longer work for
them.
But what are your 20 reasons, 18 if Premiere Pro and Photoshop were already accounted for?
(oh, avoid Project VII Dreamweaver plugins - they tell customers that the 15% of their customers who use Macs seem to call in with 85% of the problems with compatibility... that could be due to differences in Dreamweaver's being ported to OS X or it could be due to something else...)
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1. keynote --> much better looking presentations, important for sales guys
2. you can say many customers are already havings macs and they expect the same from you to get good quality

otherwise they could think your company hasn't enough money to give their employees good working machines or they think your company doesn't care.. they expect from your company to send them the best sales guys with the best equipment available. they paid much money for it blabla..
3. you are much more faster, productivity is much better with a mac
Devil's advocate again:
1. Praising the vanity of skeuomorphics?
2.
Perceived quality. Plenty of reports and articles of overheating iMacs and Macbooks spit on the claim of
real quality.
3. How is productivity faster? Blue screen of death? Have you heard of "beachball of death" that Mac users get? I've had more of those over the last year than I ever had BSODs, especially after installing Mountain Lion...
Do you have comparisons of Mac vs Windows apps (e.g. Photoshop) that run on identically-configured hardware? (e.g. a Mac, same Mac with boot camp, and any old plastic PC with the same speed CPU and other off-the-shelf items that were put into the Mac). Why should anyone take your claim at face value?