Projectile- Think of a weapon, you throw a shuriken it moves at a specific angle, movement, and trajectory. But it never changes its movement. It is no longer in control of the user either, this gives you leeway to perform another technique.
Streaming- Think of a jet of fire, you release it and it moves in accordance to the user. You have complete control over it, but the opponent you stop fueling it it disperses, as well as if you want to move onto another technique.
A projectile gives you room for combos, while a stream allows complete control over the jutsu.
Any questions?