What's the difference between open plan space and conventional layouts?

It's not really about which is "better" because it depends a lot on how you'll use it. Open plan is basically everything in one area, no walls dividing the spaces - kitchen, living room and all that integrated together, which makes the apartment brighter and more airy but creates way too much noise if you're working or need some peace and quiet. Conventional layouts have well-defined divisions, bedrooms, living rooms, kitchens separate, so you get privacy and sound insulation, but they effectively take up more space because you've got hallways and walls "stealing" square footage. It's basically the difference between having everything open and flexible versus having each thing in its own place.