Depends on the subject. For maths, I'd definitely go with exercise books because per page, they're cheaper than reinforced loose leaf paper. For content heavy subjects/subjects that get a lot of handouts of stuff, I'd use a binder so that you don't loose the handouts. But again, if you use loose leaf paper aka binders, and you're not careful, once you loose one sheet of paper, you'll miss whatever content was on that sheet.