Which is worse: using ridge vents without intake vents or using gable louvers without intake vents?

Using exhaust vents without intake vents is problematic.

Using a ridge vent without adequate intake venting could result in potential problems. First, the ridge vent may have sections that act as an intake vent in order to feed the rest of the ridge vent. In fact, the ridge vent may pull air/weather in through the front in order to feed the back. Another situation would be if a large tree was blocking one side of the house, the half of the ridge vent open to the wind could pull in through the half blocked by the tree. The side open to the wind will have a lower static pressure than the side blocked from the wind. Assuming that there is no infiltration issue, another possible consequence of a ridge vent without intake is a negatively pressurized attic. The effect here is the same as with a power fan without intake, air could be getting pulled out of the living space. Some homes are designed such that intake ventilation is nearly impossible. In these cases the best option is to go with two large gable louvers and rely on them to provide sufficient cross venting. But this is not an ideal attic airflow pattern.