While poorly sourced and potentially inaccurate, there's a chart at http://en.wikipedia.org/wiki/Damage_waiver that can help you determine what may or may not be covered by the different card association products. (And if anyone here has some concrete information they can correct and then properly source, please feel free to edit that article!)

Since I'm relatively young, I'm sensitive about keeping my insurance claim record clean to keep my insurance rates low.

On short rentals (1-2 days), I prefer to buy the rental company's LDW. I know first-hand (from both sides) the hassle involved in a damage claim, and so I prefer to pay $30 or $40 to not have to worry about anything.

For longer rentals, I prefer to use my American Express that I've enrolled in the AMEX Premium Car Rental Protection plan. For a flat $25 fee per rental (and there are lower tiers which have lower coverage limits), I'm protected with primary coverage (up to $100,000 with the plan I'm on) for damage to the vehicle. (American Express now covers loss-of-use fees; I like to think I'm responsible as a claim I filed with them a couple of years ago included LOU fees they didn't cover at the time, and I really laid into them and said that the free products offered by their competitors covered LOU fees. It wasn't long after that they announced they would start covering LOU fees.)

While I'm leery of relying on any credit card for protection because I've seen how unreliable they are, on a longer rental (a week-long leisure rental, for example), the cost/risk/benefit factors tilt the scale, for me, out of favor of the rental company's coverage.

So I'd recommend enrolling your AMEX in the PCRP for extra protection--otherwise, your card is (at least on domestic rentals) secondary to your insurance plan, forcing you to file a claim and possibly causing raised premiums or even dropped coverage, and will, at most, cover your deductible (and possibly still leave you hanging for LOU/admin fees).

If you're set on using the free products, it's hard to pick one over the other, as they all have different benefits and requirements (see the chart linked above). All things considered, though, I'd probably lean towards a MasterCard. I don't know if different tiers of MasterCards have differing coverage products or if they're pretty much all the same.