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.
Could you elaborate on the maintenance costs? We use ProtoBufjs for our own real-time whiteboarding webapp over web sockets, and in the long run having strict schemas has saved us a lot of time. We're a distributed team with different members working on the front and backends, and we frequently refer to our proto files to remember how data is transferred and how it should be interpreted (explained in our proto commented code).
Are the maintenance costs related to debugging unparsable messages? We've almost never had an issue there, so maybe we've just been lucky?
Co-author of proto3 here. Proto3 was specifically designed to make proto more friendly in variety of environments, which includes native JSON support. New Google REST APIs are defined in proto3, which are open sourced[1].
In my experience, it's not that tough to write a 'proto-to-dict' function in python, which lets you crack open the proto and look at its juicy innards...
"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.