Internally the two devices are identical. The only difference is how they connect to the Aquaero and the USB model being capable of standalone use.
The high flow sensor is the traditional model that connects to the flow ports on the Aquaero and other devices. The USB model is an MPS device that connects to the aquabus high speed port.
There is very little practical difference.
OK I think I get it, one has USB only, but both are the same. If you want to connect it directly to an Aquaero then you need the model with Aquabus.
It also appears the one with more connections has a dedicated temp sensor so can connect to a temp on the AQ6, but guessing the USB has a sensor via USB.
Just depends on how you want to read the device.
Thanks!