I think it depends.
With cash back, you know what you're getting: 1% = $0.01, nothing less, nothing more.
With a MR account, 1% = 1 MR. But since MR is a transferrable currency and Amex has their own different rates depending on how and where you redeem their points, which partner you transfer to and transfer bonuses, 1 MR could be worth less or much more than $0.01.
For example, if you were to redeem MR against statement charges, they'd be worth 0.6 cpp; if you shop with points on Amazon, they're worth 0.7 cpp. I would never recommend either, except for the occasional Amazon shop with points promos where you can use the minimum 1 MR ($0.01) to get $ off your order.
If you redeem MR for flights through Amex Travel, they're worth 1 cpp (but you can get a 25% rebate on those points if you have a Biz Gold - 1.33 cpp, or a 35% rebate if you have a Biz Plat - 1.54 cpp). So if you think you’ll eventually go for those cards to build your MR stash and use them towards flights on your selected airline, or premium cabin flights on any airline, then you should treat MR > 1 cpp.
Or you could transfer MR to airline partners to get some wild redemption values in biz and first class.
Since you now have a Plat, you can pay with points for hotel reservations through Amex’s Fine Hotels & Resorts program at 1 cpp. If you have to cancel the reservation, your points get refunded to you in cash at 1 cpp. So in that scenario, 1 MR = 1 cpp.
If you think FHR and transfer partners aren’t for you, and you prefer cold hard cash, you can eventually set up a Charles Schwab account (I think there’s a $100 bonus for opening an account too), and pick up the Charles Schwab Platinum for another 60k MR bonus (barring the pop up) for the ability to transfer out your MR at 1.25 cpp. So in that scenario, MR > 1 cpp.
Personally, I switched all my Ebates activity over from cash back to MR points, because I would never value MR at less than $0.01.
