Those two programs do two different things.... DVDFab HD Decrypter is a free decrypter that does nothing more than rip your disc to your HDD. DVDFab Platinum is an all in one ripper, transcoder and burner and is not free. The combo that many use is DVDFab HD Decrypter (free) with DVD Shrink (free transcoder) and Imgburn (free burn engine) and have no problems or cash out of pocket other than blank media.
Basically it comes down to this: money or a little extra time. Personally, i'm on the free method train.
In addition to what 7thsinger said ( the free method ) , I like the fact that I can customize with additional proggies as vob blanker, menushrink and fixvts