Solo mode mate. I've yet to play any extended amount in Online, because I find most online experiences to be painfully overcrowded with twats. Maybe this game is different, I don't know.
That being said, I'm in Aditi, doing Massacre missions (have been the last couple of days). I've made 150M in 3-4 days so far. It's also a great way to test different weapon combinations (you know, for research).