Just overall quicker experience, with trainer teleporters and such in SP I think we'll see a larger variation in stunts. Cause online you'll have to worry about the Wrong Way thing slapping you in the face, or having to go back and forth all the time. I think that's part of the reason why most stunts nowadays are so straight forward. And in SP right now you'll have to deal with the long ass loading (at least on ps3/360).
Having a trainer at hand will allow quicker, testing, say you wanted to switch vehicles, just spawn a new one and keep trying. Instead of quitting the race, restarting the race, setting up everything and choosing your vehicle. 60fps stunt vids will become the norm (I assume) and better editing tools will become avaliable.
TL;DR: Quicker stunting (teleporters, spawners) and Better videos (fps and editing)