No experience with Carte Blanche - check the specific DC board on this site for detailed info on that.

I held Diner's Club for many years, primarily because I was renting a ton of cars and wanted the primary rental insurance. I liked the flexibility to use the points with a ton of partners - used Diner's to top-off my lesser-used FF accounts just-in-time to redeem small awards. For that reason, I considered Diner's points very valuable - moreso than the typical user would because they were always liberating 15-20k otherwise worthless miles.

If you rent a lot of cars, that might be the determining factor for you. I always felt DC had the best customer service in the industry: hardly ever a hold time to talk to an agent.

Too bad ACN switched all their corporate stuff from Diner's to Amex a couple years ago...