So-so is more likely to be used to tell how someone is doing or how something turned out: Ron: How are you? Don: So-so. 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).

U can use both. "so-so" is more informal than "more or less" but u decide what u wanna use.