I'm going to chime in since at the moment we have CSR, Amex Plat personal and Amex Plat business. If I had to choose to only have one high AF card it would be CSR, hands down. The CSR is a stays in my wallet, comes with me on vacation Rick Rolled card. Those of you who remember, I was willing to wait a year to fall under 5/24 just to get that card. Instead, we went the CPC route to get it, which is completely insane! But that actually wound up being a good move in the end. So, for the practical reasons I would keep that card:
I value UR points higher than I value MR points. Given the choice to earn 3x on dining and travel between MR and UR, it will always be UR.
Paying with points for travel is easier with UR. Even when Amex business plat was offering the 50% bonus back on paying with points, you still had to have the full amount of points to pay with and then wait a few weeks to get 50% of them back. Now, they've done away with that and it's 35% back. Meh.
The $300 travel credit is easier to obtain than the travel credits from Amex or Citi since just about anything remotely travel related will trigger it and I am not saddled with airline gift cards that I may or may not use throughout the year. Therefore it is truly a $150 AF card.
CSR offers primary car rental insurance. Amex Plat does not, theirs is secondary.
CSR offers coverage for baggage delay if bags are 6 hours late at $100 per day up to 5 days. Amex Plat does not offer baggage delay coverage.
CSR offers trip delay coverage at $500 per ticket. Amex Plat does not offer trip delay coverage.
CSR offers up to $10,000 for trip cancellation/interruption. Amex Plat does not offer coverage.
CSR offesr better purchase protection coverage, up to $10,000 within 120 days vs Amex Plat's up to 90 days.
CSR has better return protection $500 vs Amex Plat's $300.
CSR offers price protection and Amex Plat does not.
CSR's Priority Pass lounge access allows you to bring unlimited guests. Amex's PP lounge access is only for the card holder and additional guests would be $27 each.
Ok so those are my reasons as to why I'd keep one over the other if I had to choose. Hope this helps