You would get 2.2 cents of value per dollar of credit card spending, which oftentimes would beat the value from using airline miles for rental cars. (milenomics.com)
But yet, books generally offer more value per dollar. (stevescottsite.com)
What it comes down to is not the type of rewards you earn, but how you can get the most value per dollar between earning and redemption. (rewardexpert.com)