… content and progress wise?
Just a few points … I don’t think you earn fans for specific events as they happen online, but when you go back to solo mode, you get money and fans for your online session. I assume that’s based on the stuff you did when you were online. So playing online does help you progress to unlocking and upgrading hubs, but it doesn’t update after every event like it does in solo.
Also, while any race you do in solo posts a time to the leaderboards, if you are racing online, those times don’t make the leaderboards.
But most of the stuff in solo is online as well. You can do all the danger signs, speed traps, speed zones and drift zones. All your remaining XP and fast travel boards are online. All of the events are there. All of your customized blueprint events and blueprint bucket lists are there and can be played with other people. I don’t think you can drive against drivatars online like in some of the Forza Motorsport games, though.
I don’t know much about the Co-op career option, or whatever it’s called. Haven’t got into that yet.
Co-op is basicly coop single-player. You tag along with whoever hosts the session, you do the races /showcases together you gain xp and fans . It’s a good way to level up and help people out! 
Thanks for your input, appreciate it!
@O Bleak Genius
When you refer to online, does that go for both adventure and free roam?