A little history might help in your decision. When Apple was considering which engine to use in making Safari they looked at Gecko, but decided to go with KHTML, fork it and called it WebKit. Their reasons for doing this was that Gecko had tons of legacy cruft still leftover from Netscape and was far more complicated.
KHTML/WebKit was newer, and thus had less legacy. It was also cleaner, quicker, and better documented.
One of the goals of Firefox 3 was to clean up the codebase and simplify it. From what I've heard they did do this, but I don't know how it compares with current iterations of WebKit. It apparently wasn't enough for Google when they made Chrome, and they have a significant stake in Firefox.
See here for more details.