i would go with the windows pc. one of my current PCs has been around for four years, without a single failure. i have managed 100s of PCs and hardware reliability is quite acceptable. while gates and jobs are getting along, you can expect apple to keep up with software such as browsers (IE) and MS office, but i am not sure how long that honeymoon will last. of course i was never an apple user, so i do not even understand user interface is celebrated as more user friendly. even if there are differences (that perhaps you have to install a driver for the CD burner on the PC; even that is not needed these days), young people are quick at figuring these things out.
and in the interest of an open society and communal effort, and all those good principles ;-), isnt it better to go with the less closed OS/hardware i.e., intel + windows?
of course all this might be moot. whatever hardware you buy, your son will probably have linux loaded on it in a year or two ;-).
--ravi