Do you want a ready to use IoT platform with storage, device registration, data visualisation, rule creation capability against data and many other futures? Then use IoT Central. It is a SaaS, meaning Microsoft will keep adding new capabilities and/or update and manage all those above-mentioned bundled services for you. Moreover, it has IoT Hub bundled with those services for you. Spoiler: IoT Central is becoming more and more flexible as you can route data to other services from this platform if you ever need to!
Do you need a service on the cloud to communicate your device to and then configure everything else (such as storage, visualisation and rule generation against data) yourself? Then use IoT Hub: it is considered as the gateway for your data into cloud from IoT devices. It is a PaaS, meaning Microsoft will support and update this service, but you will manage all the bundling with above-mentioned services yourself.
When you have a large number of devices to manage, you would rather go with the PaaS service and create your own solution, as you would not really care about the visuals too much (analytics tools are surely not meant hear). You would rather bundle the set of services that you chose (e.g. adding Machine Learning) and just stream data and get insights.