RSUs vs ISOs > before a priced round (83b is cheap at this point), and after you're a massive company, say $2-5B range (option upside is too small so you put a double trigger vest clause in to protect employees from taxes on illiquid shares)
Early stage would be plain Restricted Stock, as opposed to Restricted Stock Units, which are what is typically granted later on once the company gets large. RSUs are "units" not actual shares of stock with associated ownership rights. The recipient gets an award calculated based on the value of the stock, without actually owning stock.