IMO I like to be heavy. I have a 14' S,B, And I am sure I would go over in a lot of places had it not been for the weight in the boat. There aren't any places where you have to make a quick decision. What you want is punching power. The first 25 rapids are easy and fun. Once you go through the beginning of #2 it's pretty bouncy until you go over the edge of 3. Once your over the edge of 3 if your in the gut then your usually toast anyways. no matter what size boat ya got.
If you don't know what a class V PFD is, Perhaps a safety course is in order.