Having used both on a few projects, including a JS frontend, my advice is:

"Don't use protobufs if you don't have to".

Protobufs can be much faster, and provide a strict schema, but it comes at the price of higher maintenance costs. JSON is much simpler, easier to implement, and MUCH easier to debug. If your GPB looks like it's building properly, but fails to parse, it's a huge pain to try and decode/debug the binary. You'll wish you could just print the JSON string.

If you need the speed and schema, then GPBs are great. In our case, we got a huge speed boost just by avoiding string building/parsing inherent in JSON.