Feathers are lighter andcreate more drag than vanes of equal size; they also increase FOC, assuming the same tip weight is used. Feathers allow the use of lighter tips while still providing a minimum acceptable amount of FOC; for example, I can use a 75 gr. tip onsome feather fletched shafts and still have 10% FOC (that ain't gonna happen with vanes). If you have contact / clearence issues (non-drop away rest) or tuning difficulty feathers are the fletching of choice. I have some shafts that will not produce a good tear with Blazers, while producing a bullet tear with feathers (other Blazer fletched shafts work fine on the same bow).

Vanes retain downrange energy better, are quieter in flight and if you accidently touch them, and do not require additional waterproofing measures.

Retention of energy / downrange energy may / or may not not be a factor in your arrow selection. I've used feather fletched shafts on several deer that were 22 yards or less; I typically had about 60 LB of KE with a momentum of .41 at arrow launch. I obtained a pass through unless I hit spine or used a 2'' cut mechanical (Marden Mini Max and Rocket Hammerhead). I failed to get a pass through both times I tried a 2'' cut mechanical.If I used a conservative mechanical like the Rocket Steelhead, Wolverine (both are 1 1/4''), or Sidewinder (1 1/2'')I would blow right through a 20 yard deer.

If you want to see the downrange effect of feathers versus vanes try the calculator.

http://home.att.net/~sajackson/ballistics.htm