An option has a strike price (typically breakeven with the stock price on the date it was issued, sometimes not). After it vests, you can keep it around for a while, and decide when it's best to exercise (possibly pre-IPO, possibly after). A restricted stock unit is essentially a share of stock that they give you free as soon as it vests.

RSUs have some advantages and disadvantages. One advantage is that short of bankruptcy, they never really go "underwater". One disadvantage: you deal with tax implications immediately.

It's difficult to take either those away once they've vested, but it's unvested RSUs I believe we're talking about here.