For Guinan: I loved that her character was that classic trope of the "bartender / therapist" who was always around, but never got too involved in the main plot. She would give casual advice, but wasn't officially an "advisor", so it served the show well for the main characters to share what they were thinking.
For Quark: He was unquestionably the better developed character, with his own wants and desires that frequently intertwined with the main plot. He also served as a sort of foil to the values of the federation, which let the viewers see how greed was incompatible with the best interests of the people.
I love both. Which was your favorite? Which approach did you like better for the "bartender" character?
@Datas_Cat_Spot
Who was "better utilized" is a hard question in my view, but I think it's clear that Quark was a more fully developed character.
Guinan appeared seldom but always gave insight to characters when they most needed it. She was enigmatic, and cool, but not very fleshed out.
Quark was more realized as a character - more complex, more highs and lows, more emotions. I didn't like him much, but he was more "interesting" to watch.