So apparently they changed the textbook of Math 1ZA3 (Calculas ET) from edition 6 to edition 7. Do you guys recommend I buy a new edition 7 book or a used edition 6? Are they any different from each other?

The changes between editions are really insignificant, they just want you to buy a new book. If you see the 6th edition for a deal, I would get it, or wait till the first day of classes and ask the prof.

looking at the 1za3 homepage childs says that either edition is fine:

http://www.math.mcmaster.ca/childsa/....html#Textbook

Profs are nice enough to put up practice problems from both versions so it's really not worth the extra money you will have to spend.

Having owned the hard copy of the 6th edition and the soft copy of the 5th edition I have to say that the explanations at the beginning of the chapters were exactly the same. The only difference was in the order of the practice problems (for the most part)

Good luck with the course!

You don't need the new version at all. Quite literally, the content is exactly the same, but occasionally a topic is moved somewhere else in the book and a diagram is changed. Not a big deal lol.