The longer ski will give you more stability, most noticeable when jumping. The shorter ski might be easier to spin as there is less mass and swing weight but at your size, you won't have any troubles at all. A 180 might feel weird at first if you are used to a 171but it will take maybe 2 runs to get used to them.
Oh and remember if you are going with the 178, that isn't even 3 inches longer (2.7559") than the 171. Split that in half and you have another 1.38 inches on tip and tail.