Well if you used to ride a 178 it would be useless to go down all the way to a 171. A 181 is only 3cm more than 178...get a ruler out and see how little of a difference 3cm's really is.
If you're going to be flipping much, I'd go with the 171. Going to a longer ski than you're used to and flipping could mess you up a bit. 171 won't look tiny.