Some of you don't do your research before you post, do you?
Just to let you know, I'm not an Apple fan. I use both Wintel and OS X platforms.
First of all, Apple use DESKTOP CPU's in their iMac's and in the new 5K iMac, these are top of the range intel CPU's: core i7 they use is Haswell 4790K, or core i5 4690 in base config. They've always used desktop CPU's in iMac's.
They do use mobile GPU's, but these are high-end models, which are good for gaming too. But, you have to bear in mind that you can't use a desktop GPU in a case this small, because of heat it produces. Besides, there's no need for a desktop GPU. A mid- to high-end mobile GPU can handle photo and video editing, plus mild gaming just fine. If you want a gaming machine, build a Windows one yourself.
nVidia vs AMD: Apple will use whatever processor can meet their requirements better. If Radeon handles 5K better than GeForce, whilst being power-efficient, they'll pick that.
Apple almost picked an AMD CPU for their original MacBook air, if AMD could only meet the demand.
Now, onto the (OS X) platform in general... I've used Windows for years and have recently switched to OS X for home use.
Windows is great, because there's a ton of apps for it and majority of PC games are made for Windows only. So, for software choice/compatibility Windows is great. But, in order to make it work with every config conceivable, it's not perfect. I'm sure you know about compatibilty issues, OS slowing down over time, the OS corrupting over time etc. Personally, as I work in IT, so I had enough of messing around with Windows at home all the time.
Now, OS X. From the above comments, I presume many people aren't aware of what OS X is and what real (architectural) diffence between that and Windows is. It's based on UNIX (freeBSD to be precisE), so it's STABLE and secure. It won't crash with BSOD, because you installed some movie codecs etc.
In terms of stability, between Windows and OS X there are light years. In the last year that I've owned my Macbook, I only had to reboot it 5-6 times (OS updates and once for fun). OS X doesn't crash, slow down, or hang. It's UNIX OS, so it's super stable.
I understand that Windows is build to work on everything, hence it's not perfect. However, OS X is purpose-built, and so it works very well.
This is the main reason a lot of pro's go for it. They don't to mess around with the computer, trying to get it to work properly. They want it to just work, and OS X delivers.