The cost of a RDM is about 1.5-2c each. If you get a credit card that gives you cash back, you will get 1.5c -2c.
If you choose the DL Amex instead, you are getting 1 mile instead = 1.5-2c in opportunity cost to you.

If you choose to use it for PWM then you are getting 1c value out of it. May I suggest that you get a cash back card instead. You will get MQMs when you use the cash for travel as well.

If you use it for last minute awards, then you are getting 2-3c value per mile, which is more useful.

1 MQM is worth about 3-5c. e.g., WAS-LAX travel on Feb5-7 is 200$ = 5000 MQM.
this works out to about 4c/MQM, which is my typical ratio of value. Very rarely do I get better value than that.

I do not usually do mileage runs as my time has value too. With the double MQMs etc, one can get MQMs for 2c,
but that is entirely unpredictable, as my travel is based on my work and not the airline bonuses.

If use my SPG card, I get 1 point instead of 2c cash back from Schwab. SPG to me is 2c each in cost.
The value may be more depending on usage.
If I use my SPG to get DL miles, I get the cost of a RDM down to 1.6c each, if I use the 20k-5K bonus.

I bought all the miles I could with the DL 100% transfer bonus in April 2009 (at 1.1 c each) and
the US miles share buy gift 100% bonus - (1.35c each). I can always use my cash back card if I have too many miles.

To me 1 MQM = 2 -3 RDM or 2 SPG in cost to acquire. Also I have to travel to get MQMs usually,
so it needs a lot more effort than charging credit cards for RDMs.