I had this same problem a month ago.  I am 5'10' 180 and I got the 181, and i am so happy w/ them.  i ski 60/40 all mtn/park as well.  i knew i could handle the 181 for all mtn but was worried about the length, DON'T, they are so light and a little more progressively (forward) mounted than last years, these two things make them very easy to spin.  i am not very a experienced park skier and my first time out this year I had no problem at all spinning them, of course I was used to spinning some 185 K2 fours. seriously though, if you are a descent skier you will have no problem getting used to the longer pair.  I'm sure either one would be fine, but for me wanting something that could handle high speed carving, pow, and also go to the park if i wanted the 181 is pefect. hope this helps.  they are awesome skis, seriously, you can't go wrong w/ either.