It really depends upon what you're trying to do. If you're just talking about using it for general web browsing, then IMHO, it really doesn't matter which one you use, as Apple's quality controls on the Webkit project are pretty tight. (I've been using Webkit as my primary browser ever since Acid3 came out.) On the other hand, if you're trying to see how well your webpage works in the upcoming version of Safari, then I would expect that the current Nightly Build of Webkit would have the latest work on the rendering engine. Or, if you're trying to see what the release version of Safari 4 is going to offer, then you're going to have to go with the Safari 4 developer preview, as the new features revealed there are not yet built into the publicly available Nightly Builds. So... if there is a separate Nightly Build of the Safari 4 preview available on the Developer Connection website, then I'd say that's the one that you really want.