I'm just not sure this is the case. Joel seemed to have a much better grasp of the big picture. You could tell that he'd thought everything through really well, in the way that you'd expect from a successful business owner.
There's no doubt that Jeff did all the 'work' between the pair - Joel was mainly advising. But I would be very careful about writing off his contribution. I would personally bet a lot of money that Atwood going it alone would have been far less successful.
I'm not saying that Joel contributed more than Jeff either. When you look at two partners, and claim that one is more important than the other... that's a pretty big call to make. And unless you personally know them, I'm not sure you have the evidence to back it up.
Also, Jeff was never a bigger voice in computing than Joel.
But in terms of who has the bigger "voice", there's no comparison. Atwood is a great popularizer of tech-related topics; he writes short, sweet, lozenge-like posts that appeal to programmers as well as technical non-programmers; whereas Spolsky's focus has always been more narrow. And going by any metric you care to throw out there (Compete, RSS, Alexa, whatever) there's no comparison in terms of traffic.