Which is more appropriate: "So" or "As"?
Both are equally valid because as far as and so far as have exactly the same meaning in this context. However, so far as may be considered a slightly less formal version of in so far as.
as far as
to the extent that:
as far as I am concerned, it is no big deal
(in) so far as
to the extent that:
it was a windless storm so far as blizzards go
However, you will likely more often hear as far as I know in casual conversation. For the extremely formal, in so far as I/we know would not be out of place.
"As far as I know" is the idiomatic expression I'm familiar with.
as far as conj. To the degree or extent that: They returned at nine, as far as we know. Usage Note: As far as the Usage Panel is concerned, as far as had better be followed by both a subject and a form of go or be concerned. As far as is sometimes used as a preposition meaning "as for" or "regarding," especially in speech, but a large majority of the Panel frowns upon this usage. Eighty percent find the as far as construction in this sentence unacceptable: As far as something to do on the weekend, we didn't even have miniature golf. Eighty-four percent reject the sentence The Yankees are still very much alive, as far as the divisional race. Further, 89 percent object to as far as when followed by a noun clause, as in As far as how Koresh got shot, we don't know yet.
[The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009]
According to the same source "so far as" is a variation of "insofar as" :
so far as conj. Insofar as: So far as I am concerned, the project is over.
The rule I learned many years ago is that "so" follows a negative. For example, you would say, "She is as tall as her sister, but not so tall as her brother." I suspect this is one of those rules that few are taught these days and because it is rarely used correctly, the incorrect has become acceptable.
In my personal experience* I have found "as far as" to have a lower register (is less formal) in both British and American English, is more common in American English, and is more common amongst a younger demographic.
"As far as" is also more likely to be an initial clause in a sentence than "so far as". Thus, "As far as I know, Bob is happy" over "Bob is happy, so far as I know".
They are equivalent in meaning therefore, but choice of one over another betrays, for me, certain prejudices. I also sense that "so far as" sounds slightly antiquated and is losing ground.
- that would be northern English mining town in the 1970s to the University of Cambridge, thence the last twenty-years in CT and MA, USA.