Usually, online bonuses are sticky. You can withdraw only the bonus winnings, not the bonus amount.
A few casinos, however, offer cashable bonuses. You can withdraw the bonus along with the bonus winnings after satisfying the bonus wagering requirements.
Cashable bonuses are always better than non-cashable bonuses. However, online casinos do not say whether a bonus is cashable or not. You have to contact customer support to find out about this.