The taxes at time of exercise will depend on the gain between the strike price and the FMV, known as the spread or the bargain element.
important If you’re granted ISOs or NSOs at a low strike price, and the bargain element is zero, then you may be able to exercise at a reasonable price without triggering taxes at all. So assuming the company allows it, it makes sense to early exercise immediately (buying most or all of the shares, even though they’re not vested yet) and simultaneously file an 83(b) election.
caution An 83(b) election, as already discussed, is the choice to be taxed on the receipt of property even though you might have to forfeit or give back the property to the company. You can make an election on the receipt of stock, but you cannot make the election on the receipt of a stock option or an RSU because options and RSUs are not considered property for the purposes of Section 83(b).
caution ISOs are often preferred by startups, as they’re supposedly better for employees from a tax perspective. This assumes that (1) AMT won’t be triggered and (2) you’ll get a low long-term capital gains rate by holding the stock for the appropriate holding periods. However, often you either run afoul of the AMT trap, or don’t hold the stock long enough with the complicated 1 year + 2 year requirement, or the spread at exercise is small or zero, so the difference wouldn’t matter anyway. NSOs do have a slightly higher tax because of the need to pay employment taxes on NSOs and not ISOs.
controversy Overall, it’s not clear the ISO is that much better for employees, so many people argue for NSOs instead.
confusion This is partly because ISOs can make it harder to meet the long-term capital gains holding period.* Many people expect early exercise, together with an 83(b) election, will help them hold the stock long enough to qualify for long-term capital gains. While this is true for NSOs, a murky part of the rules on ISOs states that even with an 83(b) election, the capital gains holding period does not begin until the shares actually vest. So if you want to immediately exercise an option and file a Section 83(b) election, and you might have liquidity soon, it’s better—for those who can—to do so with NSOs.