I want to buy a 1TB external disk. Should I get HDD or SSD?

I have my PC and it has a 640GB HDD. I want to get a back-up drive so I can store my files and documents. I think it's good to go with a +500GB disk. Should I get HDD or SSD? And which one is faster in reading/writing data? Are SSDs more expensive? And what is a good brand/model? Thanks
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