I'm not sure post-spending is the right thing to look at. One of the many reasons working at a startup is an awkward sell is that it means giving up money during your lowest-earning years in hope of getting more in your higher-earning years. That's the opposite of the smoothing function a rational person would prefer. That extra spending may well have provided a large amount of extra utility.
As far as taxes, a big difference you didn't mention is that startups most frequently exit in a single liquidity event that can't be deferred (i.e. acquisition). Getting that $1MM in one tax year is much worse than spreading it out over even 3-5. At least in my locality, the difference between taxes on a $1mm windfall in one tax year vs. a senior dev salary at a place like Google is ~5%.
> Getting that $1MM in one tax year is much worse than spreading it out over even 3-5
Not if it's long-term capital gains. The year that I banked most of my gains from my company's acquisition (sub-$1m, but just), my tax rate was substantially lower than in previous years. Paying taxes like a rich person has distinct advantages since the game is rigged in their favor.
Another difference could depend on the nature of the individual earning that money. If that individual is disciplined and reasonably good at investing their money, taking the corporate job, living frugally and investing everything that's left might make them come out ahead. But if they're like most people, earning more will make them spend more and they'll come out behind the start-up employee who will usually immediately invest most of the windfall (either in a home or the market).
Neither route is obviously better, but it's probably worthwhile to look at the post-tax, post-spending bank balances of both sets of employees to see who comes out ahead since it's not a simple as comparing $1m to $1m.