The emerson jpc comes with plastic plates, they're hollow so they are not as stiff as real SAPIs but having a piece of plastic pushing on your spine isn't exactly pleasant..
You can fill them with sand and they become as heavy as the real deal iirc..
These foam ones (just noticed they're from emerson as well lol) are thicker but they flex a lot more so the carrier bends and molds to your shape instead of digging into your back. They're also slightly heavier (talking grams) than the plastic ones but you don't really feel the difference once wearing it.
I see. The MODI came with foam plates and has always been super comfy to wear for skirmishes.