1.) Why buy a Mac rather than an x86 box?

Apple hardware is just great. I have a 17" iMac, and the display is superior to any other LCD screen I have ever seen (at ANY price). At 1 GHz, the G4 processor outruns a Pentium-III with twice the clock speed. And it´s a perfectly silent system, very much unlike the clattering and hollering of normal x86 boxes -- not to mention the fact that it takes away very little desk space. I guess you might get x86 systems with similar specs, but they are pretty expensive.

As an afterthought, there are also very few viruses using the PPC instruction set.

2.) Why run Linux on it rather than OS-X?

Both KDE and GNOME are superior to the Mac environment. Whether they look better is a matter of taste, but you can make them look just the way you want. Besides, they come bundled with lots of applications. For me as a LaTeX addict, Kile is the thing to be keen about. What kind of LaTeX support is there in OS-X? TeXshop (downloadable from 3rd party sites, not included)? Forget about that.
When you try to port genuine Unix software, OS-X is really nasty. E.g. EMBOSS, the open-source tool suite for molecular biology: That one compiles right out of the box on Solarix, Irix, AIX and Linux but requires lots of tinkerings and additional downloads on OS-X.
Apple´s X11 compatibility is, at best, poor. It is slow and not properly integrated into the system. You can´t just run X11 programs from the Finder, it needs some ugly AppleScripting (as with OpenOffice). I did not manage to get Xfig set up on OS-X -- and it´s still the f***g best tool for drawing illustrations for scientific LaTeX documents.
In general, KDE´s integration of applications and "managers" is much better than that of OS-X. How do you set up icons and programs to be started under OS-X? In KDE and GNOME, you just select them. Ever worked with the OS-X version of Emacs?