Here is the short answer: if you need to connect Bluetooth and/or Zigbee devices to your Wi-Fi network, buy the Nexohub; if your are implementing a Linkstyle Smart Security system, buy the Multi-Function Smart Security Gateway.
While the Nexohub Smart Multi-Mode Gateway and the Linkstyle Multi-Function Smart Security Gateway have some functional overlap in their Zigbee connectivity, they are specifically made with different use cases in mind.
The Nexohub is a simple, general-purpose hub that supports 3 different protocols: Zigbee, Bluetooth, and Bluetooth Mesh. Its sole purpose is to connect those types of devices to a 2.4GHz Wi-Fi network so that they can be controlled over the internet using the Linkstyle app.
The Multi-Function Smart Security Gateway is specifically made to be a security hub with Linkstyle app integrations. Using this gateway, you can quickly arm and disarm your smart security system, which cannot be done with the Nexohub. Furthermore, the Smart Security Hub has a built-in speaker that can function as both a notification chime and as an alarm siren, which the Nexohub does not have. Lastly, it also has a built-in LED night light, which the Nexohub also doesn't have. However, the Smart Security Gateway only supports Zigbee devices and cannot connect Bluetooth devices to your Wi-Fi network.