I actually have both PMTS and Sofa Ski School and worked on them extensively so I can shred some lights.
I first bought SofaSkiSchool in early 2010 when I was wedging and struggled to make any parallel turn. People just told me to "turn" but how? There wasn't any clear explanation on how to make a ski turn. It was like magic.
Klaus' DVD introduced me to the use of edging by turning the knees to put skis on edge, come up and forward during transition. After some practicing, I was able to carve long-turns in a park-and-ride position but was out-of-control. It wasn't pretty but at least I can make parallel turns.
However, I quickly realized edge-locked carve was simply too fast for my skill level and put me in in danger in steeper terrain. I wanted to ski slower but SofaSki school didn't have a lesson for that. As result, I took a lesson with a local ski instructor which probably wasn't too useful except that I finally got having some MA videos.
After posting on this forum, someone introduced me to PMTS so I googled it. I bought the Essentials and DVDs which were quite good. It introduced many concepts which I didn't know before such as creating an angle or turning is done by tipping the feet instead of rolling the knee(SofaSki). I was able to improve on my tipping ability still struggled with uncontrollable "carving" until someone recommended How to be an Expert Skier 1 & 2 & DVDs.
Essentials and DVDs are great for advanced skiers but if you want to learn PMTS, How to be an Expert Skier 1 & 2 & DVDs is the way to start. The books follow a linear progression model and cover everything from the very basic of eliminating the wedge to bump skiing. Essentials is like a reference book, and How to be an Expert Skier is like a training manual.
I'm thankful for Klaus' DVD because it introduced me to parallel skiing and gave me the first feeling of carving but PMTS propelled me to the next level. I'm still an intermediate but I get comments and questions from people while riding on chairlifts such as "are you in a race team?", "can you teach me how do you make those turns?", "you have good technique". The most sincere compliment is when I see others including instructors began to replicate the exercise I was working on.
Skiing seemed like magic to me until PMTS came along because it broke down every movement into simple and understandable term and have drills to perfect those movements. PMTS made me a faster skier with better balance yet able to dial down the speed at will. PMTS also made me a versatile skier. Today, we had 30 cm of powder and was quickly turned into bumps. Luckily, I had been practicing Uphill ski balance and the super-phantom move in the past weeks or else I'd have struggled severely under today's condition. Maybe there are other methods but PMTS is the only comprehensive system I've used that teaches excellent technique in a way that could be practiced by anyone.
If you have the choice, start with PMTS because it will(with a lot of practice) make you a better skier.