The answer is simple when you look in terms of the same currency:

Example: $10,000 in spend at a Marriott/SPG Property on a:

• Marriott Chase Card = 50,000 MR / 16,666 SPG
• Ritz Chase Card = 55,000 MR / 18,333 SPG
• SPG Amex = 60,000 MR / 20,000 SPG

In every case, paying with the SPG Amex makes more sense and I no longer need to keep this heavy metal Ritz card in my wallet.

Of course, this is from a redeemable points perspective. If you're chasing status through Ritz Card spend or Marriott Card nights/points earnings then YMMV.