I have to agree with Equestriancowgirl4 I prefer Parelli, partly for the reasons she mentioned, I've seen both work with horses in person and when I went to CA's clinic he got (my opinion) very hard on the filly he was working, I've been to 4 Parelli clinics and have never seen Pat or any of his students treat a horse too harshly(again my opinion).
But I don't really want to get on the subject of what constitutes a beating and what amount of physical pressure is necessary to teach the horse. Probably every clinician has videos circulating of them "roughing up" a horse, and they also have given responses as to why it was needed for that horse. My reason for choosing PP is the results I have gotten from each program.
CA worked great on some of my horses, but three of my horses couldn't handle the amount of pressure and the speed at which CA recommends you apply it, they became very spooky around me and learned very slowly in that program.
PP explains how to tailor your training style to meet each horse's needs as an individual. I know many people find the whole "Horsenality" thing confusing, but found it to be very enlightening. It finally made sense why certain techniques worked on some of my horses but not others and how to decide which techniques will be effective for what horse. So now I watch a wide variety of trainers and I can pick out what tasks will work most effectively on each of my 5 horses individually.
What drew me to Parelli in the first place was his high level students. I feel that almost all of the clinicians are great at teaching horses, but PP was the first clinician that I seen show what he could teach to a human. Another clinician that has turned out some very skilled students from his program is Denis Reis. I have yet to see a CA student that really REALLY impresses me.