Yes, it's AMEX SPG and gives more choice where to convert points. SPG points earning is 1 point per £1, M&M Diners/Mastercard is 1.25 point per £1. If you transfer from SPG to airline in batches of 60,000 then you are getting 1.25 miles per £1. I realised that flights that are most interesting to me, are cheaper on United. The same flight on M&M is 40K miles or 25K miles on United. So, it's much more efficient to transfer points to United than trying to earn more on M&M.
I plan to spend my M&M miles this year and not extend M&M card, no point to keep paying for it. SPG is much more useful and I can transfer to M&M if needed. I don't fly Lufthansa too often, so I'm not at risk of collecting miles that will expire in future. That's the only reason I applied for M&M Diners cars, to keep my miles alive and 10K miles bonus was decent.
Of course, there's major difference where you can pay with AMEX and where you can pay with Mastercard. However Mastercard can't be used on petrol stations as it's pre-paid and I haven't try to use Diners there.