In my years at BigCo I worked with engineers who have 10-20 years more experience than I do. Often felt like the dumbest guy in the room. I learned a ton. No comparison. But I would never have gotten the job there without my previous startup experience. And without that experience I'm not sure I would even have been able to learn from my BigCo co-workers so successfully.

I got tired of corporate politics and a high traffic commute, so now I'm happily back in startup land.

There's huge value to getting the lay of the land.

It is true that it's easier to be stagnant as an associate at a larger company. If you're not interested in taking on new challenges you can generally sort of fall through the cracks. However, folks who are proactive at a FAANG style company can find great mentors and support to develop skills really quickly. Often skills that simply aren't available to people at startups...

What's more valuable: spending your formative years working on systems at really high scale, or building a quick RoR MVP and managing 3 VM's in AWS for a startup?

You may never get to manage a cloud VM. You will probably never get to help decide how a CI build pipeline works, how engineering hiring should work, etc etc.

I'm not discounting your point about learning to develop software at scale, but your overall scope of responsibility is much narrower. It's a tradeoff.

BigCo titles are much more narrowly defined, so most of the time you can't even see the parts that might have new challenges...or cracks.

I learned a lot from that job and it opened a lot of doors for me in my career.

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I disagree. They have, necessarily, a greater willingness to accept risk since they are already in a risky situation by definition.

If anything the startup is likely less tolerant of failure compared to a big established business, in that it has nothing to fall back on and will simply fail and go out of business.