MPX is
another app entirely! Since Amex killed 5x MR with the Plats on MPX, you have to do a little thinking to decide (1) whether to use MPX vs. Gyft/Swych, and (2) what's the best card to use with MPX.
1. With Gyft/Swych, you currently earn 3x UR when using CIP or 5x UR when using CIC through PayPal checkout. That's 3x/5x on every card you see on those apps. Very easy to figure out.
2. With MPX, you need to look at the merchants whose gift cards are available on the app, then think of their merchant code (Starbucks=restaurants/dining, Uber=travel,
Amazon=merchandise?). Does the merchant you want to buy a gift card for fall into a bonus category on the credit card you want to use?
For example if you want to buy a Starbucks gift card, you'd use the CSR to earn 1 mile/$ (+25% bonus if you have a MileagePlus credit card connected to your MPX account) + 3x UR/$1 because Starbucks triggers the dining category bonus (or if you're using a CIC, then that's 2x UR/$1). Likewise with Uber, you'd want to use a CSR (for the travel category) or CIP (travel) to earn 3x UR/$1 and 1 mile/$ (+25% bonus if you have a MileagePlus credit card connected to your MPX account).
For merchants who don't fall into a bonus category, you might just want to use a card that earns a high point value on every dollar spent, like the CFU or a SPG, and earn at least the miles for that merchant on MPX. But then you have to decide, is 1 SPG point or 1.5 UR + the UA miles > 3x or 5x UR through Gyft/Swych?
ETA: For example, I need to buy dog food from Petco, but that's not in any bonus category. I could buy a Petco gift card on Gyft in $10/25/50/75/100/200 denominations and earn 3x UR with CIP or 5x UR with CIC/Ink+. Or, I could use MPX to buy a Petco gift card in the exact amount that I need at the cashier, earn 5 miles/$1 through the app (+25% bonus miles because I have a MileagePlus card added to my account) and earn 1 SPG point/$1 because I used my SPG Biz. I usually choose MPX in that case.