I did paper tests for my practice and took the CBT...I think paper was harder simply because of the extent of time that you had to stay extremely focused. I was completely drained after the CBT and had a 3-day headache so strong, I could barely sleep the first night. I can't imagine having to go through double that!
I was wondering this too, since you have to make fewer mistakes to score 10+ on each section. Though, since it is shorter, there fewer chances to make such mistakes. I will have a stronger opinion after I begin doing some full lengths.