Generally speaking, double taper lines are easier to mend and, depending on the skill of the caster, can land on the water more delicately. Weight forward lines are easier to cast for distance. The other advantage of double tapers is that they can be reversed after a few seasons, so you are getting two lines for the price of one.
Thanks for the info
Personally, I prefer DT lines for dry flies; it allows me a better roll casts, and smooth presentation over the water surface, what else can I say, more "natural". Oh, thanks for the info about Cabela's lines, very useful.