they're not even in the same league!
Let´s say they have a different focus. Joerg Strohmayers OWB is just a browser. Its as bare bones as it gets - no bells, no whistles, while FABs OWB has many more features and keeps adding to them. But AFAIK they are pretty much the same when it comes to how the websites look.
The common part is more than just WebKit. WebKit doesn´t even include a Javascript engine IIRC. But things like bookmarks, windows, filemanagement, etc. are certainly not part of the OWB-base.
