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?