As far as I am concerned they are the same in that regard. And since both of those tools are not built by "developers", but by "frontend engineers" it doesn't really matter.
I spent like 15 minutes trying to figure out how to attach Firebug to the browser window. Only to find out that the upward pointing arrow in a gray circle means attach to browser window. On WebKit I click an icon that looks like a browser window with developer tools.
The resources tab in WebKit is ten times better then the mishmash of CSS/Script/Net/Cookies tabs all spread out.
Anyways that is just my opinion, and as far as I am concerned Dragonfly was the best.