From what I learned despite "typically" consumer WD drive have better durability than Seagate (you can dispute that with data), the surveillance drives both are very durable. I would not be worried about saving money using either brand.
As to 256MB vs 512MB, they likely are for buffering data before those SMR / CMR tracks are written all at once. My understanding is it has more to do with the bit density than how fast you are writing. I would just use drive size and price as deciding factor for which drive to buy.
Those drives last a long time. I think my dad's system with 4 security cam has been on for 5 years on a WD Purple and 8 years on the original system drive with Toshiba Enterprise and still going strong. They are optimized for reduced seeking, just fast enough rpm, and therefore reduce wear compare to typical desktop drives.
