I had a feeling they weren't exactly comparable in specs -- otherwise, why would the prices be so close (knowing Sigma's general business model). However, I think the SB-600 would work for most people who aren't doing portraits as a business.
There are other differences worth at least a mention -- one being that the Sigmas have a plastic "foot" and the Nikon's foot is metal. I suspect the Nikon might handle a bit more force before snapping. I had an aftermarket flash years ago (as film was beginning to wane) that I really liked, but it got hit (not that hard) by a football that took a bad bounce. Snapped clean off -- and the plastic foot was the weak point. Now, I can't say a flash with a metal foot would have fared better, but I can't say it wouldn't.
I've got nothing against Sigma, and if I needed the higher power and other features at a lower price than the directly comparable Nikon, I wouldn't hesitate. I ran the comparison the way I did because I already have one SB-600, and I don't want to ask the university to pay for two flashes when I really only need one. It may not be true, but the idea of working with two different speedlights -- different makes, models and power -- makes me uneasy. And the fact remains that I can buy another SB-600 for no more than I'd spend for the Sigma, and I think that the Nikon flash would meet my fairly simple needs just fine.