Well, your ordinary wet kit, has one jet that is usually tapped infront of the TB, the nitrous-fuel mixture amount that goes into each cyl. is not even because the flow of the mixture is traveling with the vacume from each cyl.
Direct port, is a jet tapped in each port of your intake plenum, directly infront of each intake vlave, (sprayed directly into the cyl.) thus the same amount of nitrous-fuel mixture is delivered evenly to each cyl. These kits are usually used when running a large 100+ shot with a small motor, forged internals, rods, pistons, ext... Direct port is nice if you have a built motor and you plan to run large amounts of nitrous. Its a more safe way to deliver large amounts of nitrous. (usually with a built motor)
If your looking to just spray 75 shot and down, I would recomend a basic wet kit (one jet infront of the TB) it's less involved, and you will see nearly the same results as a direct port with 75 shot. (with stock motor)
NX kits are the best IMO. All they sell are wet kits, direct port, and basic.