"That $375k would then be subject to 63% taxes, netting me $138,750, or a 20% down-payment on my piece of the american dream. The other company does the right thing, so I only see a 15% capital gains tax"

A bit of an aside but could someone elaborate on this? Is the OP referring to an 83(b) election? If so, isn't that his responsibility (of course, the company should advise him to do this). Or is this referring to something else?

At any rate, my impression is that there would be California taxes as well on that income, bringing the taxes well above 15%.