intellectual1 the solomon papers are definately harder. The whole point of them is just so that you can have questions given that are all harder than what you would actually be given. So they will give you the hardest it can get with logarithms, the hardest it can get with integration, diffrentiation, trig etc..
the reason for that is because if you can confidently do those with high marks then you can definately do the real thing. Because solomon is the hardest questions that you'll get on each topic, so if you can do those, then you can do whatever edexcel is gonna give you.
However, it can be a bad thing too. Some people do get demotivated by if they can not do the questions in the solomon papers. If that does happen to anyone. Then just stick to the edexcel ones. These are just if you find what edexcel gives TOO easy and you want to make sure you can do the harder questions. If you're not one of those people that finds what edexcel TOO easy, then you're better off sticking to edexcel papers and making sure you can do every question they've given in a past exam paper.