I have both cash back cards and Amex. I've had 200k+ miles racked up at times and then switch to using cash back card. If I had to choose I would go with Amex for me. I usually take 3 trips a year and use miles.
I just looked at a round trip for this Christmas time. Buying the ticket I would spend $416.00 that's on our website. Take 20% off that if you do "fly confirmed for less". Same ticket with same departure times is 17,000 miles. That's cheap and there are tickets listed round trip for 9000 miles, even cheaper. For several past years I've been able to get round trip tickets with miles way cheaper than buying with cash. I estimate to me that the Amex perx is worth over 1200 bucks to me in travel benefits. Cash back deals don't come close to that dollar amount. And that's running just over 50k on the card a year.
In addition to the above, I also get one companion certificate a year as well. That adds another easy $400.00 So using the above fare of $416 add spouse or whoever for $70 total. You now have 2 tickets for the price of 416+70=$486
I think the important take away from this is that skymiles, if you have skymiles, it opens doors to cheaper tickets. Whether you use miles only or both cash and miles. My destinations tend to run +$400 a ticket. If you only buy two tickets a year and they cost $200 each, maybe the Amex card is not for you.
Also important note. To get the most value out of your miles, obviously you want to be booking months in advance. However, I have gotten last minute deals for not so many miles.