It really depends on exactly what reward you want.

First - you need to peg the value to something so determine what flight and class you want as a reward - this will give you the number of miles you would need to redeem.

Second - research the cost price to buy a ticket on that route and class.

Third - divide the 2 to work out the value (in cents) of an individual mile.

Once you do this you can directly compare cards for different airlines (which might have different award prices for the same route/class) as well as cashback cards percentage rate per dollar spent (since this is directly comparable with the number in 3 above).

Of course you also need to look at the earning restrictions with the cards (annual and monthly limits etc). But it's a start. Blue cash gives cashback on 'year to date' spend so the higher 1.5% rate doesn't apply for the first part of the spend per year.

You can get a better rate than MR rewards for airline mile cards as well...