We looked at both and chose Render based on various posts here along with our real world testing and bake off parameters. We use the Pro Ultra config and handle ~20rps on a Node / Express stack w/ 36 workers using clustering. Our bandwidth is around 50Gb / month.

Render has been quite solid and the support has been on point when we have found issues or run into unexpected edge cases. I have been impressed that despite a big raise and subsequent scaling up, they continue to ship a solid product and the platform improvements have been useful as well.

Never used Fly.io but I have a website running on Render (2.4M unique users per month) and the service is getting quite nice. Occasional hiccups, but they are improving. Not as stable as Heroku used to be 7-8 years ago yet, but it's also waaaaay cheaper.

I really like their Docker support and infra as code, makes it very easy to spin up a whole thing while not being too far conceptually from Kubernetes for example.

Running production app on fly. There have been some hiccups but generally we're quite happy with it.

I have services running on each. Fly's more flexible. Render's further from the metal, and thus somewhat more limited.

Both have some reliability issues and rough edges. My new stuff is mostly on Fly, but I'd use Render where I had a junior team who had zero infra chops.