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 slightly less formal, as it short for 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
You will likely often hear so far as I know in casual conversation. However, you will probably only see as far as I know in written/formal contexts. For the extremely formal, in so far as I/we know would not be out of place.