SO:
I think that you will be comfortable with 161S, which will be easier to control and which will better correspond with your weight. I think that you don't really need more edge than 161S will give you. Gaining 2 more kilos in your muscles should better your overall carving ability at first, and will not really poor a shorter board's performance at all. If it does so, you won't be able to spot it. So finally: As you won't really change your physics -> I'd say go for shorter board.
The most often we do ride with LOTS of other people on crowded slopes. This means that the shorter the sidecut, the better to steer clear of others.
Last word to say:
168S will be better for overall performance, with longer edge, better grip but will demand better technique. Again this is very subtle not really big thing.
161S will be better for usually met conditions and will give you most of fun more often.
This is my opinion. Anybody thinks different, please write it ! I'm not trying to show that I know everything
