"Son of a bitch" carries gravitas accumulated over centuries. An asshole is just an asshole, a hapless chump, a pointlessly obnoxious hindrance. An asshole can be an evil schemer, but he or she can't be a worthy opponent. If an asshole gets over on you, you feel dumped on. If a son of a bitch beats you, you can live with it. You've probably learned something.
... Hold your nose and say "asshole." Sounds like, takes one to know one. The second syllable, if it stood alone, would sound expansive, like whole, soul, roll, from pole to pole, and goal. The emphasis, however, is on the first syllable: that flat-to-nasal a and the hissy two s's.
"Son of a bitch," on the other hand, starts with a little hiss but goes on to rhyme with gun and then luvvah, rolls over a solid b bump, picks up that tight-tough it -- as in git, hit, and chickenshit -- and then comes down on that great crunchy ch sound, as in you betcha, snatch, crotch, and hootch-a-ma-cootch.
Elsewhere in the issue: more on the A-word, the last word on douchebag¹, and an appreciation of faygeleh. The print edition has an impressive chart of the relative frequency of various epithets, 1996 through 2006.
¹ With an unfortunate apostrophe error in the intro. R.I.P, copy desk.