I haven't seen a lot of the ads, but your brother's assessment of most of them ("but I've found even the toughest ones to be addressing one of two things: legitimate advantages of the Mac, or legitimate disadvantages of the PC") is pretty far off from what I've observed. Only a few of them actually "address" actual real advantages or disadvantages. In fact, I'd say FUD characterizes
most of these ads: suggesting that the PC is going to destroy your data or get a virus or some other horrible thing. The rest of them are pretty much making fun of Vista, but not in a "here are some legitimate problems" sort of way so much as a "hoh hoh hoh, that Vista is
wack, yo" sort of way.
It also sort of irks me that the ads are characterized as Mac vs. PC. It's really MacOS vs. Windows, since the criticisms are all leveled at the Windows OS. In fact, for the past several years the computers that Apple sells are what were a long time ago called PC-compatible (now all 'PC's are PC-compatible, since PC was an IBM brand and IBM doesn't make consumer systems anymore). In fact, most Apple computers can boot Windows or Linux with no problem (and some computers from other manufacturers can boot MacOS, though it's fairly rare; if you are interested see
http://www.hackintosh.com/).