I know absolutely nothing about the Atomic. But I do have an '09 Promise, so I can speak to that a little. I also wonder why you're limiting yourself to these 2 boards?
I'm not sure I'd consider the Promise a beginner board. lilfoot, can I ask what about it makes it a beginner board? Though I haven't had the fortune of riding a NS which is undoubtedly stiffer, I still think the Promise is slightly stiffer/more responsive than a board I'd want to hand to a beginner.
I'm no advanced rider by any means, but I'd consider myself intermediate. I don't touch the park, but I like messing around doing flatland stuff, ollie-ing off rollers and small bumps. I also like cruising or bombing the blues, and especially love that flying feeling when carving.
I'll go down blacks if my friends want to, but otherwise I don't like taking the gamble of finding myself on top of a mogul field or ice covered slope. Haven't tried any double blacks yet.
I'm pretty happy with the board thus far. My only regret is getting a 151 instead of a 154. It holds an edge well on icier conditions, is fairly responsive, and has pretty good pop.