Ridge Vents vs Gable Vents Energy Savings as a Percentage or $ Figure
Neither is necessarily more or less efficient. Attic ventilation is a calculation of cubic inches of airflow, taking into consideration soffit venting and outflow.

There are lots of online calculators to determine this. Here's a link to the first one I found, from GAF:

https://www.gaf.com/Roofing/Residential/Products/Roof_Vents/Ventilation_Calculator

Depending on your area, ridgevents have just become a go-to for ease of figuring. That doesn't make it right, but it isn't necessarily wrong either. Down south where I live, you can't vent your roof too much. Up north, where heat retention may be an issue, proper venting may assist your attic insulation.