Personally, if I’m buying new devices, I would buy zwave plus with S2 security.
Z wave plus was a huge feature advance Over the older generations in several ways. Range, power utilization, and explorer frames which allow devices to be paired in place. All things an end-user will notice And benefit from when setting up a device.
Additionally, association changed significantly with Z wave plus and you can’t even make associations with some of the older devices And keep the zwave plus advantages.
S2 is important because only S2 devices can repeat S2 messages.
I don’t think you need the new series 700 devices unless they have specific features that you are willing to pay for. They do have even longer range and even better power management, but it’s not as much of a comparative advantage as Z wave plus versus zwave classic.
I’m fine with keeping any zwave Classic devices you already have, But if buying new, zwave plus is definitely worth the usual price differential.