They both make holes. A spade or paddle bit is about like a chainsaw, where a Fortsner bit is more like a table saw with a good blade. If you just need a hole, not very precise and spade bit will do, but if you need a consistent diameter hole with smooth sides, use the Fortsner.
If you are running wires through floor joists, that might be were you would use a spade bit. If you are making holes for dowels or round tenons that's where the Forstner would be best.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)