The difference between these two phrases is very, very subtle. You could use either in any context and be understood, but "until now" perhaps suggest that you are about to reveal what you remember right now, and "so far" may suggest that you may do so in the future.
"I haven't studied much so far..." suggests: "I haven't studied much yet, and I may or may not begin studying soon."
"I haven't studied much until now." suggests, "I wasn't studying, but now I intend to/now I am."
I hope this helps!