At a $90k salary, you'll feel the pinch more in NYC. Expect to spend a significantly larger portion of your income on rent in Manhattan compared to Santa Monica. Factor in higher costs for transportation (NYC subway vs. LA car dependency, including gas and insurance), groceries, and entertainment. While California has state income tax, New York City has its own city income tax on top of state and federal taxes. Run a few online cost of living calculators, but generally, you'll have more disposable income in LA with that salary.
honestopinion: Living expenses in NYC will eat into your salary much faster. LA offers a better financial cushion at that income level.