Most OEM carbs utilize CV ("constant velocity") design, where engine vacuum is used to operate a spring-counterbalanced slide that acts to maintain a constant rate of airflow through the carb throat. The throttle grip directly controls a butterfly valve in the intake tract, but much of the actual throttling is done by the CV slide.
"Flatslide" carbs provide a direct connection between your right hand and the opening of the carb venturi... there's no equalization or damping based on what the engine can handle; what you ask for is what you get. This direct-acting throttle is great for race engines but does require a bit of finesse for riding on the street... open the throttle too far/too fast at low revs and the engine may bog badly.
There's tons more involved but that's the nickle tour.
"Better" is going to depend a lot on how you ride... the CV carbs give really good results when dialed in and are less demanding for street riding; the flatslides place more demand on you for throttle control but can be set up to give exceptional throttle response on a bike that is able to take advantage of them.