This also brings to mind a question I've been ruminating lately as to why the seeming demise of the -ess in words that take the -or suffix. Especially, now it seems that actress is getting jettisoned in favor of actor.
In fact, the new AFI Awards renamed the category "Best Female Actor in A Leading Role" in lieu of "Best Actress." This befuddles me, because I've never heard actress deemed denigrating to any degree from the feminist viewpoint. In fact, to me actress always seemed relegated to a higher pedestal than actor, thus "Best Actress" being traditionally saved as the final, and most important, individual category at the Academy Awards. On the other hand, why do we say doctor and never doctress?
But I've also heard the -ess suffix applied to certain -er words as in hunter/huntress (especially when talking about the animal world, as in lioness...hmm, well there you are...female lion?)
Which brings us back to the determination of the -er,-or in the first place...perhaps an "adviser" is more generic, and "doctor" and "actor" is more specifically descriptive of a professional person. However, I'm sure there's plenty exceptions that can be found to all of these examples. I like Faldage's very credible proposal...but, there, too he found an exception almost immediately. Although it seems like there must be a common root to all this somewhere.