Relevant discussion here...
http://flyertalk.com/forum/showthread.php?t=650442
...the thread title, and the first few messages, will probably seem irrelevant but keep reading.
Also, here is a snapshot comparison of Diners Club Rewards versus American Express Membership Rewards...
http://dinersclubus.com/dce_content/...ewardscompared
The comparison is slightly out of date, Amex now has 15 airline partners.
To be sure there is no misunderstanding, Diners Club in North America is now a Mastercard, acceptance is universal. However, I see that your primary airlines are CO and UA. These carriers no longer participate in Club Rewards, but Marriott does.