the 171's will be easier to ride for you and probably more fun. But as the AR6 is farely narrow under foot it will compromise you being able to ski powder. If you only want to ride park then this won't matter.
However if you are slightly more focused on big air/ big kickers then rails you should get the 176's......more ski to handle but it would provide you with better float and control on big airs.........
my suggestion is because you are fairly light go the 171cms...