I think you'll end up spending more for aftermarket, but you'll get a more versatile system. The bars themselves are cheaper, but you need to by feet too to attach them to the factory rails.
The factory bars, at least on my '10 (the '07-'09 racks are different, but I think have the same problem), have very large plastic ends on them that limit how much width of the bar you can actually use. There is about 4" of extra plastic at each end. I put bikes up on my rack and getting all 4 up there is a squeeze because I can't use the full width of the bars, I had better luck wit the rack on my Odyssey. If you're using it for a cargo box, the factory bars should work fine.
Here's 4 bikes squeezed on the roof of my Outlook: