I would assume FVector is ideal, as an impulse could be represented by a vector in space with a certain magnitude. The FRotator would be useful if you need to keep track of the orientation of something. I guess it would be possible to combine both the FRotator and FVector to create an impulse that both pushes an object and gives it a spin. This is at least what pops into my mind on the subject.
Maybe it’s best to ask, what do you want to happen when the object receives the impulse? That could change the answer drastically.