Hard to answer. Both progs are good. dvd decrypter is no longer update so you can't back up new sony ARccoS unless you use a litle help from the psl plugin. but on the other hand dvdfab can. for me i like to use DVDdecrypter + PSL plugin work everytime.
DVD decrypter also is more than a ripper unlike dvdfab decrypter. It will rip whole dvd or just the movie, create an iso or file & it will also burn. I prefer decrypter over dvdfab decrypter but like soberfree said since it's no longer updated it may need help. I haven't tried the psl plugin route yet but use AnyDVD with it (changing a few settings in decrypter)