We use both but personally i think cambridge has a greater range of questions dealing with different situations which is why i prefer cambridge-also the questions they give are closely related to the examples they give-so cambridge 3U would be first choice for me. wish Pender had written a four unit book as well-Arnold is good but the difficulty levels aren't as high as u'd hope for 4U.
I like the variety that Cambridge has and the slight repetitiveness of Fitzpatrick. Excel has some nicely explained examples but lacks comprehensive excercises.
Yeah, I agree with bboyelement, the extension and last few development questions are more equipped for 4 unit students, many of them fall into the harder 3u category of the 4 unit course.
Maths in Focus by Grove is good to start off with... it has easier problems and better explanations..
then move on to Fitzpatrick 3Unit book and you'll be set..