I go with dollars back. That said, YMMY.

Dollar-denoted rewards are harder to devalue than miles or points. That is the main reason I prefer cash back. As a rule-of-thumb one values miles at a penny/mile. So your choice is 1% in cash or 1/2 % in miles.

Now, can you leverage reward miles to be worth more than a penny? Yes and no. On the yes side, can you accumulate enough miles to get status? And can you get rewards such as class upgrades that are worth more than a penny a mile?

On the no side, United is pretty stingy about reward availability. Plus your "free" seats these days come with some pretty large fees. Christopher Elliot's column is very negative about airline loyalty programs. And he singles out United for criticizem.

Applied to my own travel and spend levels 1% cash yould be a far better deal than 1/2% in miles.