It's your choice.
Coverage offered by the rental company makes a huge profit which by definition means the amount paid out to repair damage is a very small fraction of the money collected from renters.
(a) The coverage may "add 50%" or even "double" your total rental price.
Coverage offered by the rental company is actually better than what most credit cards offer and what you get from your own insurance company provided yhou don't violate the rental agreement (like drive on dirt roads or leave the keys in the ignition).
(a) The rental company coverage is primary i.e. it pays first so your own insurance is not involved, whereas credit card coverage is secondary to your own insurance which may cause you to be surcharged. (Some Diners Club cards offer primary car rental collision coverage)
(b) The rental company coverage also covers diminution in value to the car following repair that you must pay for (you know, 'they' can never fix something as good as new) whereas you may have to shell that extra amount out personally otherwise because that is not part of the 'repair bill' per se.
Disney hints:
http://members.aol.com/ajaynejr/carrent.htm
I decline the rental company coverage and rely on my credit card.
Before signing the contract and going to get the car, read carefully to be sure you have declined or accepted exactly the parts of the coverages you don't want or want. And double check the preliminary price to be sure it reflects what coverages you have declined.