For two reasons, which will answer both of your questions:
-First of all I use a mobo because I simply don't have enough space to install a desktop here. iMacs on the other hand are take up very little space since it's just the screen (which contains all the hardware), the keyboard and a mouse. The laptop originally came with Windows vista, but after two days of using it I concluded it was the biggest pile of bull**** I've run on any computer in my life, I threw it off and installed XP. However, sometimes the OS decides to become insanely unstable (yet my scanners never pick up any indication there might be a virus at work), and this, in conjunction with the fact that the mobo's video card has got massive overheating problems, leads often to a whole load of problems that tend to pop out right out of the blue (like the BSoD problem, which has been happening today, after working perfectly for a year). I'm not sure if it's the OS' fault, because this mobo has some weird and stupid hardware problems, for instance, when the screen is tilted more towards to keyboard, the power cuts, forcing the computer to switch off the unhealthy way.
I'm not sure how Mac's OS wouldn't be very good, as I personally only heard people praising the system for its stability. Some friends of mine graduated in graphic design, they used both Mac and Windows when they started their studies and now they all stick to Mac, stating that Mac never gave them any trouble in all those years. In the one year I've been owning this thing, I've been having problems with it more often than not. During the year I've been using this thing it has undergone at least three full system formats and re-installations. I'm tired of having to put up with these instability issues and therefore hope that I'll find a much more stable system in a Mac computer. Because honestly, this computer, while it should've been a so called "top-of-line" notebook, feels more like a sinking vessel to me.
-The second reason is that I'm going to study photography. The curriculum will include graphic design as well, so the switch to Mac is only logical to me. On the gaming side it won't be an issue to me. The only games I play on computer are those from the Unreal franchise, and Blizzard games except for WoW. I'll keep using this computer solely for level design purposes, as that will become quite harder with a Mac.