If you are not into modding saws then I would saw the 2172 as there are more of them out there for parts down the road and a better resale value down the road.
I would buy the one you feel most comfortable with.
As far as I know, there are no differences apart from the limiters in the transfer covers of the 2166. They really made it too easy to upgrade the cheaper saw this time, and gain quite a bit of power.
The same is of course the case with the Husky 365XT vs. the 372xpXT.