Most people who have experienced at least studying for FAR and AUD will probably say FAR is more difficult. Then again, there are others who cannot pass AUD. Everyone ranks degrees of difficulty differently. However, based on the sheer amount of information in FAR, it can be a bear. Then again, AUD is mostly about remembering rules and which rules apply to which type of Attestation/auditing engagement. FAR is based much more in calculations, and for many of us accountant type minds, remembering formulas is easier.
But let me give you the best advice...it doesn't matter which one is harder because you do not get to pick and choose between which parts you want to pass. To be a CPA, you've got to pass all four parts within an 18month window, plain and simple. The real strategy is how long do you want to study and which parts should you take first. I suggest really thinking back to college, and study how you studied then. Do not lie to yourself if you are a procrastinator and say you will study 2 hours a day for 3 months. For me, I need to schedule an exam and put the pressure of knowing a test is upcoming and then I can force myself to sit down and learn in a much more compacted schedule of about 4 weeks.
Of course, if you're trying to judge how hard each part will be to you, think about how you did in your college courses. If you aced auditing and struggled a bit through intermediate/advanced accounting...then far will probably be much more difficult. For that reason, you may want to take it first and hope to get it out of the way, or you may wait until last so you can tell yourself its the last part and give yourself more confidence.
AUD-Failed (Waiting for Score)
BEC-PASSED (First try)
REG-Failed (Retake February 2013)
FAR-Failed (Retake January 2013)