RSU: taxed on vest date* as 100% of the market price is the basis (since the "exercise" price is $0.00)
Option: taxed on exercise date** as the basis is the market price minus exercise price
* "exercised" / owned automatically on vesting date
** the option should be vested on or before the exercise date. Vested option should not be exercised immediately but before expiration (usually 10 yrs)
Lizzy, Flubber, it may depend on how these things are set up, but I do the taxes for my son who works for Intel. He gets those grants and when they vest (not sure of the right word, but when he gets stock) a portion is automatically sold to pay for taxes. As a matter of fact the IRS rules for his particular situation is the amount added to ordinary income.
They have another program for him where he can buy stock twice a year at a 15% discount and any of those that are later sold do get calculated as a capital gain/loss. But the grant
When I was receiving options or RSU's that matured they would issue me only the % amount over the taxable amount and the remaining were shown as sold. I never received the full amount.