Obvious disclaimer: I'm not a tax specialist. You should not rely on tax advice found in an internet comment. I probably got some stuff wrong below.When you exercise an ISO, it is not considered regular taxable income. It is as if you legitimately purchased the stock on the market for that price. No tax is due until you sell. If you sell immediately then you pay regular income tax on the gain. But if you hold it for at least a year, then you pay long-term capital gains rate, which maxes out at 20% (vs. 37% for regular income tax).
When you exercise an NSO, the gain is considered income and is immediately taxable at regular income tax rates, whether you sell it or not. If the company is not public, then you can't sell share to pay the taxes, so if you're going to exercise NSOs, you'd better do it either before the valuation has gone up much (in which case you end up paying mostly long-term capital gains tax), or wait until after IPO (and pay regular income tax). Since investing in startups is very very risky, exercising early probably isn't the right choice for most regular people.
Extra complication: The US has two tax codes that exist in a sort of quantum superposition. Each taxpayer must pay whichever tax bill is larger of the two. The above describes the regular tax code, but there is also Alternative Minimum Tax (AMT). Under AMT, ISOs are not special; they are treated like NSOs. But, the maximum tax rate under AMT is 28% rather than 37%, and doesn't kick in until higher income levels. So if you exercise ISOs before the company has gone public, but after the valuation has raised significantly over the exercise price, you might again have trouble paying the taxes. But if you manage to pay them, and you manage to hold for a year, your overall gain will probably be 10%-20% more than what you would have gotten with NSOs.
Extra extra complication: Let's say you exercise your ISOs before IPO and you manage to pay the AMT. Years later, you sell the stock. Is your capital gains computed based on the ISO exercise price, or the valuation at exercise? BOTH! Under regular income tax, you owe capital gains on everything since the exercise price, but under AMT you already paid for the gains between the exercise price and the valuation at exercise. If you don't want to get double-taxed you have to learn what it means to take a credit for a timing-based AMT adjustment. This is well beyond the point where Turbo Tax gets pretty unhelpful and you probably need to get a CPA, but in the Bay Area there are so many rich clients that a decent CPA will charge thousands (maybe tens of thousands) of dollars to do your taxes. Have fun!
All that said, if your plan is to exercise-and-sell in one action sometime after IPO, then I think ISO vs. NSO doesn't make much difference.