I have long been a fan of WebKit, if anything because the Developer Tools. After spending so long with Safari it took me a little while to finally give Google Chrome a chance. Switching browsers is an extremely large life decision can't just be made so easily. Chrome quickly became my default with its speed, auto-updating of the browser and extensions, ability for one tab not to crash your entire browser and of course still being WebKit, the awesome Developer Tools.
Besides the many other great reasons Chrome is my browser of choice the speed and always updating Developer Tools are along the main. WebKit is in a constant flow of development, including improving their Developer Tools, and Safari updates just don't come as often as you might like. Chrome on the other hand pushes out updates every time you turn around, although thanks to its auto-updating, you barely notice. This means the Developer Tools are always improving. Not to mention how pretty it looks vs Firebug.
Paul Irish posted a video of 12 Tricks for Chrome Developer Tools a little while back that I just came across. It's a great video if you're new to Chrome Developer Tools but also I also picked up a few things I wasn't aware of. Lots more information can be found on Google's page for Developer Tools.