I think it is personal preference for what kind of experience you want.
As a founding engineer you have complete control over how things are built and who knows, you could be the CTO or a VP in 5 years if this goes well. What is certain though is you would need to take care of everything by your self or with a tiny team for a couple years. If that sounds fun, and it does to me as someone who want to launch my own start up, then go for that! You will be getting paid to be on the frontlines of learning how businesses are built. There is a 90% chance the startup will fail but a 100% chance you’ll learn a lot up finding product market fit and iterating to get there. With 5-6 years of experience and a job offer in hand + the Taro community you can crush it in that role. But there will be little structure that you don’t create yourself.
As a Senior SWE on the platform team at a late stage start up, the advantage would be that from day 1 you can be shipping to likely millions of users. If you like the team you interview with that would be a huge bonus knowing you have awesome colleagues to collaborate and learn from. At the startup you would be hiring the rest of the team yourself. There would be much more regular routine and structure than with the startup. If it sounds better to not be the sole person responsible for production failing or succeeding this would be a better option. At the same time the engineering challenges would be focussed on very different scalability concerns vs trying to find product market fit and building a bunch of stuff that gets no users. You would do a ton of learning new tech at the start up on your own to take care of everything needed vs building on the best practices of the team and company at the large startup with a bunch of peers.
They both sound like great options, and if you put in the work with the bigger picture in mind you can impress your next hiring manager with your stories either way.
What are the pros and cons associated with picking a founding full-stack engineer role at a pre-seed startup over a senior SWE role in a platform engineering team at a decently high-scale late-stage post-IPO startup?
I'm going to assume you meant pre-IPO as post-IPO means it isn't a startup anymore haha.
At high-level, the pros/cons here aren't that much different than startups vs. Big Tech. Look at Stripe. It's not public yet, but it's bigger than a lot of public tech companies (e.g. it's 5x bigger than Zillow) so it might as well be a Big Tech company and it suffers a lot of the same issues.
In a nutshell, the smaller company has higher potential upside with money and growth while the bigger company brings consistency with money and growth. What you choose is up to your personality and risk tolerance.
Here's some other good resources on the topic as well: