Many people on BOS dislike Excel study books because they find it too simple and easy.
I haven't used them myself, so I can't give you a personal opinion.
For 3U, it's seems people on BOS like Cambridge. Once again, I haven't used it myself.
Most of the times, if you buy a book labelled "3 UNIT", it would include both Mathematics and Mathematics Extension 1 questions (3U books generally separate the MX1 questions from Mathematics questions with an asterisk or highlight (you will need to look at the first few pages of the book to find out since it differs from book to book). So no, you don't need to buy 2 books.