both solutions will work. The Gateway + Extensions will allow you to distribute the devices across the building, thus having a "tree with branches" kind of topology. The pure Neuron will result in the "tree" topology, where you will have to have longer cables from the PLC to each device/sensor/switch/light/etc. From SW point of view, there is no difference. The Neuron's however can be purchased with RPI4, so you can have more performance and RAM.