I agree with you that Terry Pratchett is so much better. But it’s not very comparable. Harry Potter lives from the comfy feeling of a magical boarding school and the classic “chosen one” thing. It’s easy reading and draws a very clear imagery in the minds of the readers. The disc world is a bit more… Abstract I guess?

So I’d rather pirate Harry Potter if it’s something that interests you. And definitely also read Terry Pratchett, simply because it is better.