I haven't heard a lot about the nug, but what I have, it wasn't so great. First of all it offers v-rocker, which is rocker between your feet and at the tip and tail. Second, it is a channel binding system, so if you want anything other than burton bindings, you need to see if you can get the baseplate adapters. Ok so the reason the guy told you a 150 is like a 158 is because this board has elongated contact points, which means that the contact points were pushed out further than that of a normal 150. Also the tip and tail are shortened. The best way to understand this is to put it up against a 158 and you can see that the contacts points are nearly in the same place, but the 150 just has less tip and tail.
I have a very similar board, the nitro swindle. I ride a 145 and I'm 5'7 190. I rode this board all of last season and I loved it. It could pretty much do everything, except powder. I had many a front flip from nose dig, but it was expected. The difference between the swindle and the nug is that the nug is rocker, where as the swindle is zero camber. The ride for me is fast, nimble, and poppy(could use more). Either way, depending on your snow conditions, it may not be the best mountain board, since thats what it sounds like you wanna do with it.