Apparently, both Exams are similar - although most find the 1st a bit more difficult as it is a bit more stressful. This is according to Sally-Anne - we were discussing it on another thread.
The first paper is often cited as being more difficult than the second. There seems to be no explanation for that other than the feelings of stress, and both my DSs scored higher marks on the first paper than the second, despite declaring them to be the harder papers.
Inevitably all the children will feel that the "real tests" are harder than practice papers, because they are in a test environment and there are no parents or tutors nearby for reassurance.
I will not be surprised if your DC all come out of next wek's tests saying that "it was much easier". Of course, if next week's tests covers a lot of question types that your child doesn't enjoy as much as others, they may conclude that next week's test is more difficult!
Both tests are historically very equal in difficulty - it is a matter of circumstances and taste.