I'm not sure which HDD i should buy. WD Black 500 GB expensive than WD Blue 1 TB in my country. If Black worth to buy it i would get Black. I really need a fast drive. 500 GB storage is enough for me. I don't know how much read and write speed difference that 2 drives.
If you really need speed a SSD is highly recommend. As for the drives speed isn't really the difference between the WD Black and Blue it's more reliably, quality control, and other professional oriented features. That being said the Black is slightly faster but no where near what you'd experience with a SSD