I realize that this post may be coming a little too late, but I'm going to throw it out there anyways. In my opinion, you could really go either way on this, and it ultimately depends mostly on your "trajectory." What I mean by this is, how do you see yourself skiing next year? Are you looking to further improve your skills and keep pushing yourself as a skier? Or do you expect to be skiing in pretty much the same way as this year- moderate speeds on East Coast groomers.
If you think you'll be skiing in mostly the same way next year, then the 172 would be a perfectly fine choice. That ski would be a completely comfortable cruiser that you could take at your own pace, and not have to worry about being on your A-Game to handle. On the other hand, if you're looking to progress your skills, then I'd push you to get the 180cm. The added length will give you a bit more stability at higher speeds and in the crud. At the same time, I don't think you'll find the extra 8 cm too inhibiting, as the Soul 7's are super lightweight and the sidecut ends well before the tip and tail (shorter turn radius).
Final thoughts: You'll be good either way, but the added length would help you progress your skills if that's what you're looking to do.
Hope this helped!
Matt @ Skiessentials.com