What are the tradeoffs between a single HID descriptor/endpoint with multiple reports vs multiple HID descriptor/endpoints each with a single report?
They serve different purposes
Multiple HID descriptor allows listing different devices, like a mouse and a keyboard, custom within the same USB connection.
You could have for instance a device that lists a custom device with your own driver as well as a COM port directly supported by the system, allowing several communication methods.
Endpoints are really different communication endpoints.
Concerning speeds, the limit will be the communication speed of the USB device and USB protocol. Endpoints are a few bytes of data sent when the device is connected, nothing really that will make any differences.