a mac is a pc, running osx and/or/instead of windows.
difference between 825 and 855 is basically just the lcd screen, price tag, and "gamer" marketing demo. if you want extra monitoring gadgets on the router then get the 855, otherwise there's no point, 825 is practically the same thing just cheaper. personally I would rather use software to monitor those kind of things, so I got the 825.
look here if you want to compare features, you'll find they're virtually identical save the flashy lcd screen:
http://www.smallnetbuilder.com/component/option,com_chart/Itemid,189/