Replying to 171 or 178 Line Invaders 08' (Help)

Hey guys, I was just wondering what size would be better for me with the Invaders... I'm like 5' 7" and I weigh about 140lbs. I've been snowboarding park for several years and now I want to take up skiing and like the last time I skied was when I was 8 or 9 years old. So I was thinking about getting 178s but would they be too big for me and would I lose any flex because it's a longer ski? And plus I'm planning on skiing all park but sometimes outside the park onto like normal trails. So 171s or 178s I gotta know.