I've used both quite a bit. They both aren't bad, but the RO seems a lot more consistent as far as size and quality go. I have found plywood in Cowboy before.
Also, I light my BGE with a torch, and Cowboy tends to spark a ton more than RO. Lots of dust, which I assume is from the break down of the lump during shipping and stocking.
Around my parts, RO tends to be a bit more than Cowboy (unless there is some kind of sale), but I'd say the couple bucks more are worth it as the RO seems to last longer, burn cleaner, and has less dust.
Also, and I don't know if this really indicative one way or the other, but I have seen fire-grilled (or whatever they call themselves) using RO for their grills. Don't know if this is just because that's what Sysco carries or if they actually prefer it. Usually restaurants favor consistency above all else (including taste).
Naked Wiz has a great charcoal comparison guide by the way, though most probably know that. http://www.nakedwhiz.com/lump.htm
(Is it cool to give other sites props here?)