The majority of the time, if the data is coming directly from the devices, either directly or via a field-based gateway, IoT Hub will be the more appropriate choice. Event Hub will generally be the more appropriate choice if either the data will not be coming to Azure directly from the devices, but rather either cloud-to-cloud through another provider, intra-cloud, or if the data is already landing on-premise and needs to be streamed to the cloud from a small number of endpoints internally. There are exceptions to both conditions, of course.
Both solutions offer very high throughput data ingestion and can handle tremendous streaming data volumes. In fact, today, IoT Hub is primarily a set of additional services that wrap an underlying Event Hub.
Read on for more scenarios and limitations in each. They definitely serve different use cases.