I disagree if money isn't the primary issue. I think the best part about Checkpoints is that they organize the questions into the topics they belong. For example, in Physics, there's a chapter on Basic Motion, Forces, Energy, Momentum, Projectiles, Circular Motion, Gravity and your investigation. After covering each topic, you can just sit down and do some questions, whether you do some half an hour a day, two hours a week - it doesn't matter. What matters is that you're doing them and you're familiarizing yourself with the type of questions that are expected to come up. Whereas if you do past papers, you'll either have to know the content already or you'll have to just flick through the paper until you hit a question you know the theory behind it (even then, some questions combine knowledge across a whole range of topics, some of which you haven't learnt yet).
Before the exam you'll want to do past papers since you'll probably know all the theory required so you can become more familiar with the pacing of an exam within the time limit. But Checkpoints is useful for studying throughout the year. Plus, I think that it's more motivating and encouraging to yourself by doing a few questions a day from a topic in Checkpoints than open up a past paper and going through it to see what you can do.