All else being equal the smaller gap will produce more drag and steepen the glide slope. The material I've read in the past indicated that a smaller interplane gap results in more drag due to more airflow effects between the wings. A larger gap reduces this effect. Like most things it's a sliding scale with no one totally correct answer. After all some of the last racing biplanes actually had very small interplane gap ratios so there must have been other factors at work as well.