You shoudl definitely be riding a 160. As snowolf said, NS boards have a longer effective edge than similarly sized boards from other brands. For instance my Heritage 158 has more effective edge than a Nitro TG 159 that I used to have.
15 LibTech TRS HP . 15 NS Snowtrooper . 15 NS Ripsaw . 13 Salomon Pow Snake
14 Now Drive . 13 Now IPO
14 Flow Hylite