Prometheus developer here. Paul is right that Prometheus is and will always be float-only (strings are possible in a limited fashion via labels), whereas InfluxDB supports many data types. I'd presume the query languages are fairly similar in power in practice (Prometheus is Turing Complete).
Another Prometheus dev here. Yep, Prometheus itself doesn't aim to be a durable long-term storage. But in other ways, its scope is bigger and more about active systems and service monitoring: from client libraries (which don't only speak some metrics output protocol, but help you manage
e researched Prometheus and while it required to rewrite queries it now ingests 4 times more metrics without any problems whatsoever compared to what we tried to feed to Influx. And all that load is handled by single Prometheus server, it's f