I'll be trying to grab resources for building my garage, so I'll start with campaign. The logical progression order in my head is learning > racing, so I'll probably go Driving School, Circuit Experience, and then Missions. Once I've earned a few cars I'll probably be heading into into the livery editor and photomode intermittently, maybe pop into some open lobbies if I have any creations I'm particularly proud of.
I had plenty of time to try out Sport Mode in the closed beta and I know I'll be getting a lot of use out of it once I perfect the career modes, so I'm in no huge rush to hop back in.