It really has extremely little to do with what textbook you use. Of course certain textbooks like Maths In Focus do not have very challenging exercises (although their test yourself and challenge exercises are great for topic revision) but this is to ensure the basic skill is mastered first before moving on to challenging questions. These more challenging questions should be found in past papers rather than any text book.
Sometimes the Fitzpatrick text book does not allow for the easier questions to build the strong foundations of the basics, although other a fairly decent set of questions. Terry Lee is much like the Fitzpatrick text book in terms of difficulty, although I find the layout can be slightly off-putting.
In terms of a well rounded series of questions comprising of an increasing difficulty, Cambridge is by far the most suitable. It has the questions ordered in an increasing difficulty and classifies them into different categories. The extension questions offered are generally fairly difficult, whilst the beginner questions are rudimentary. It has a massive amount of questions, so finishing the exercise isn't the goal but you have to decide when you've done enough of one level to move to the next. However, sometimes the Cambridge book can be too repetitive, even with different difficulties.
It won't increase your chances of getting a band 6 (E4 in Extension courses) because they all do have different advantages over each other. Besides, your better off just focusing on questions from past papers for any of the harder difficulty rather than relying on your text book.
