NightShift is currently limited to NetFlix while PlayOn supports a larger number of services. Obviously, both also require that you have a subscription for those services beyond what they charge.
NightShift is a hardware based solution requiring a compatible router (selected Asus routers or they offer a starter kit pre-configured) and therefore doesn't require that your computer be on while recording.
Playon is a Windows based software solution.
Both now allow scheduling for free zone periods (recently introduced with PlayOn)