This has to be the most nonsensical post of the week. I get what you're trying to say, but you obviously have no idea what you're talking about. The Webkit nightly builds use Safari. So... How can Safari's engine be better than Safari? Safari uses webkit, webkit uses Safari. Without Webkit there wouldn't be a Safari (and it could be argued that without Safari Webkit wouldn't be as good as it is)
Safari is older, more stable Webkit, as said.
Webkit is simply a faster, more powerful, if slightly less stable version of Safari - however I must say that Webkit has not crashed on me whereas Safari has several times.
Note: I just did an Acid3 test, just to see how Safari and Webkit (trunk 35773, 15. Aug nightly) stacked up. Safari 3.1.2 scored 75/100, with LINKTEST FAILED, whereas Webkit managed a perfect score. I believe Safari 4 (installed on my other computer) also achieves that, but again, the current build of Safari just isn't as fast as Safari 4 or Webkit.
So, I'm planning to use Webkit for the time being. Firefox 3 is okay, and I use it occasionally, but WK is my day-to-day browser.