Does anyone know when to choose http2 vs websockets?

I'm assuming you mean HTTP/2 with Server-Sent Events, since raw HTTP/2 frames aren't exposed in the browser?

My answer would be use HTTP/2 + SSE whenever you can get away with it. The primary limitation of SSE in this case is you can't natively send binary data (you would have to base64 encode it or something). If you're just using JSON or another text format anyway this isn't an issue.