Fitzpatrick explains thing in a more reader friendly way. I understood it alot more clearly reading Fitzpatrick book first. Personally, feel that alot of students studying Extension for the first time would get a bit lost with the terminology used in Cambridge if they were doing certain topics for the first time.
Cambridge explains things in a more theoretical way. Cambridge explanation is more closer to HSC standard and university style maths.
I think in the Foreword of Cambridge they do mention that the purpose of the book is not to do every question. (They make a suggestion to do as many questions until you understand the concepts) There are 3 phases: Beginning, Development and Extension questions. If you knock off the majority of the Development questions it should be enough to get an E4. If you do knock off the extension questions, then an E4 is a certainty.
It would not be a stupid thing if you went to an easier text say Grove as well as they explained Binomials quite well , circle geometry as well.
I hardly touched Circle Geometry back in my high school years, so have been going back to the 5.3 texts (year 10) to brush up on some aspects as there is an overlap with extension 1 Year 11 maths.