Math and reasoning question: Is there a difference between paying for SUB with rent with fee versus organic spend if you also have a separate3% rewards credit card?

I have a Robinhood Gold 3% credit card and just opened an Amex Platinum with SUB. Obviously I’m going to try to organically get the SUB with travel, but in case I fall short:

Is there any real difference between paying the remainder of the SUB with rent with the Amex for 1x and paying a 3% fee and using Robinhood for daily spend for 3% …

Versus

Using Amex for 1x on daily spend and keeping rent with 0% fee to achieve the remainder of the SUB?

I feel like mathematically, assuming 1 CPP, the strategies are equivalent, but am I missing something?