Just buy whatever is cheapest... or nothing at all. Chances are you wont even use it that much because of the abundance of lecture notes/tutorial notes available at your disposal. This is ofcourse not sound advice for people that want to do every question they can get a hold of, but instead, people that just want a reference book or a book they can practice a few questions with.
I know a few people in 3rd year that haven't bought a textbook at all.
I know a few people in 3rd year that haven't bought a textbook at all.