gc85, Eric S and neonorchid,
In my opinion: As gc85 is only 120 lbs, the 164 is the perfect length for skiing bumps, trees and powder. The 172 will be a lot of fun on groomers, especially at speed, but I think it's too much ski in all other conditions for someone that only weighs 120 lbs. The only people you should be listening to are those close to your weight, not those that weigh 170+ lbs.
As for me, at 140 lbs, if I skied 25+ days a year, I'd buy the 172. But I'm lucky to make 3 trips out West and get in 12 days. If the 172s are a lot for me to throw around, they're going to be a lot harder for someone that only weighs 120 lbs. And yes, neonorchid, it is all about snow conditions. FYI, I just bought the 164s yesterday. Hope I made the right decision!!