There is no difference in 'quality' what so ever as they are both rippers not transcoders.
DVDDecrypter does a lot more functions. And is an truly excellent program. Except it will not remove the latest copy protections on DVD's (anything produced after January 05 as this is when the last version was released). DVDfabdecrypter will remove them as it is constanltly updated.
If you wish to only use DVDdecrypter, you could do what i do and that is buy AnyDVD and use the Elby .dll in DVDDecypter which makes it fully up to date and perfect to use once again. As Anydvd works in conjunction with it to remove the latest disk protections. They make an amazing combination when set up correctly using the two alongside each other.