The neck/chin thing is a sloppy guideline, at best. I'm 155lbs, 5'10" have a size 9 boot and ride a 147cm. With that being said, I'm a rail-rat. I hit a few jumps, but nothing crazy. I also like to bomb the hill a time or two, but my mountain has a total of something like 900 vertical feet. For what you'd like to do it'd seem the 150 would do. The 156.5 would seem long in MY opinion, but I'm (extremely) biased towards shorter boards. Do you know what length you were riding before?
life IS good.