Saying that BJJ is simple is incredibly false. You could train BJJ for over 10 years and still feel like a complete beginner when training with other black belts. There is an infinity of techniques out there, it's a never ending sport.
That being said, I would be tempted to say that BJJ is more complex, but that simply because I train BJJ and I never tried Muy thai, so I won't.
If there's one real way to find, go train BJJ and Muy thai and you'll be able to compare them.
One last thing, are we talking about no-gi or gi? You couldn't even imagine how more complex the gi game is. My teacher always tell me that no-gi is kind of like "checkers" while gi is a "chest" game.
When we go to the ground, you are in my world. The ground is the ocean, I am the shark, and most people don't even know how to swim. - Rigan Machado