Unless I'm missing something, you'd still only have a "2 point" strobe system with a second SB-600 ~ as the onboard flash, when set to commander, does that & only that - sure, some of the onboard strobe ends up in the image, but it's a minimal amount that you can't control.
You can do the same thing with a second SB-800, but you can also, should you wish, have the 800 fire both a commander burst for the 600 and strobe as normal itself. Then you get a two point system with on-camera & off camera flash, something you can't do with two 600's. And as previously mentioned, despite reading in most places that the 800 isn't worth it over the 600 in terms of features vs. extra cost, I've really, really liked the 800 whenever I use it (my own strobe is a 600).
This is all a moot point if you don't actually think you'll use two strobes that often, as then you are better off going for the most budget route.
