Both! The insurance provided as a standard card benefit is for damage to the rental car only, and is secondary to any other insurance. In other words, you file a claim with your personal auto insurance and any amount they do not cover, such as the deductible, may be reimbursed.

The Premium Car Rental Protection is "primary insurance for theft and damage to your rented vehicle, you won't have to dip into your own insurance first (which could lead to higher personal rates), or pay a deductible" and it includes other types of coverage, such as Accidental Death or Dismemberment, Medical Expenses and Theft of Personal Property.

Only you can judge whether these are worth $25 per rental, but if you own a car this probably duplicates coverage that you already have and it's a matter of whether you have a problem filing a claim.