A 3/4 zip jersey hits a nice middle ground between full ventilation and a cleaner fit. The zipper runs about three-quarters of the way down from the collar, so you still get solid airflow when you unzip, but the lower portion of the jersey stays seamless and smooth against your body.
The main advantage is less zipper bulk at the waist. Full-zip jerseys can sometimes create a small bulge or stiffness right at the hem where the zipper ends, which is noticeable when you're leaning forward on the bike. A 3/4 zip eliminates that, giving you a slightly cleaner, more comfortable fit around the waist.
The trade-off is you don't get quite as much extreme cooling as a full zip wide open. For most riding in moderate conditions, that's rarely an issue. But if you ride in very hot climates or do intense efforts where you want maximum airflow fast, full zip still has the edge. Go with 3/4 zip if you value a smooth waist fit and ride in temperate conditions.