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.