Yesterday I flew an older A320 from MXP to CPH, then a neo from CPH to ARN (seat 2C on both flights). No meaningful difference in seat pitch or width that I could discern. The neos have the newer interior with USB power ports in the seats. They also have big winglets and the engines sound a bit different. If you are already Gold and have FastTrack and Lounge access, Plus and Go are only differentiated by service level (including free food and drink) and mileage awarded, with no center-seat blockage or physical changes in the cabin. Additionally, Plus is always the forward-most rows in the cabin, so it effectively blocks the chance to book an exit row seat for additional legroom.