Ah, I didn’t realise the USB one cost that much more. I’m not sure most people would prefer the USB version though. It’s convenient to move around and you can use it with mini PCs, but cooling isn’t as good compared to something that sits in a case with good airflow (so it’s more likely to thermally throttle while in use), and having dedicated PCIe lanes as you’d get with an M.2 is way more efficient than using a shared bus like USB. Google have always advertised the USB version for “prototyping” while the M.2 versions are for “production”.

For $40, you can get an M.2 version that has two Coral TPUs on a single board. https://coral.ai/products/m2-accelerator-dual-edgetpu. I’ve got this one with a PCIe adapter, but currently only use one of the TPUs.