Pellets release their fragrant oily goodness faster than leaf hops, so they are better for whirlpooling and dryhopping (IMO). I've used both many times and I never get the same aroma when using leaf hops late. You don't need to weigh down pellets, since they sink. Leaf hops float so it's better to bag them with marbles or metal washers. Weighing it down with spoons, or large objects can cause problems when you try to remove the bag from the narrow neck of the carboy. Another issue may have been with the kit itself. These kits regularly recommend topping off with plain water, using more than 7% crystal malts, and dry yeasts, "some" of which accentuate malt and mute hop character and do not attenuate well. All bad things for an AIPA.
Amarillo is not tropical. It's fruity, citrusy, floral, and peachy with mild grapefruit notes. This would be an excellent late addition hop or dryhop, as would Centennial, Simcoe, Citra, Nugget, Cascade, Summit, Columbus, Chinook, Horizon, Ahtanum, Galena, or Sorachi Ace in pellet form. You have to use enough of these to make it count. 1 oz. for dryhopping 5 gallons is simply not enough for an AIPA.