Apart from the SMR vs. CMR, if your NAS will run 24/7 you need to make sure to use 24/7 capable drives or find a way to flash a 24/7-specific firmware/setting to a consumer drive. Normal consumer drives (e.g. WD Green) tend to have a lot of energy saving features, e.g. they park the drive heads after a few seconds of inactivity. This isn’t a problem with normal use as an external drive that only gets connected once in a while. But in a 24/7 NAS the drive will wake up lots of times and park again, wake up, park again … and these cycles kill the drive pretty fast.