I think a lot of it is really how engaged a rider wants to be.
A good example, one of my main riding buddies is my dad, im 25, 155# and in fairly good shape ( I ride 50+ days a season) and he's 51 180# and gets out maybe 25 total days a year.
with these comparisons assume 2 people riding together and the same tree'd hillsides/creeks/etc
For me, I enjoy the 155 class sleds. They definately give up some in the straight climbs and require a more active riding style when in technical terrain. Being shorter and more nimble it sort of just wanders around, floats the skis and is generally more "playful". I love this riding style. It suits how I want to enjoy my day, fast and aggressive. The sled reacts quickly to my body language and allows me to make split second calls on my line choice.
for my dad he enjoys the techincal terrain but prefers a little slower and more calculated technique. He wont take all of the wild lines the younger guys do and he doesn't want to deal with being stuck all the time as its flat tiring. the 163 class sleds work great for him as he can still go everywhere we do as far as general area to area access, but the sled is alot more stable and calm so he can pick and chose his route and not be hoping around like a mad man.
IMO, whichever riding style better describes you is how i point someone to a 155 or 163.