Honestly... it's both m8.
I've just gotten lvl 40 myself by doing a combination of, remote racing, grinding the dream car championship (mainly to cycle the UCD and for some credits), and doing the 4hr and 9hr events. 4hrs during the day, 9hrs over night.
But the issue is, if YOU don't remote race your friends guys, then THEY aren't going to remote race YOUR guys.
So you need to balance it between all of them I'd say. Take a day of running remotes like crazy so your friends know that you're putting in your time, and then do a bunch of B-spec races. That way, the bobs you're racing are earning you xp, and the bobs ur friends are racing are earning you xp. It's like you're double dipping!