So-so is more likely to be used to tell how someone is doing or how something turned out:
Ron: How are you?
Sam: Hey, how did that lasagna turn out?
Pam: It was so-so.
(Here, it reflects quality; neither of these examples would use "more or less")
Jim: Do you know Russian?
Tim: More or less—I understand a lot, but have difficulty speaking.
Sharon: Did you finish that report?
Karen: More or less—I just need to add the references.
(you can't substitute "so-so" for either of these. "More or less" refers to degree or quantity).