Here is the math, assuming you convert to airline miles:

$20,000 = 20,000 Club Rewards = 20,000 airline miles (Transfer fee $19)
$20,000 = 20,000 Membership Rewards = 20,000 airline miles (Transfer fee varies)
$20,000 = 20,000 Starpoints = 25,000 airline miles (No transfer fee)

Diners Club Rewards accumulate at Citi, and there are 20 airline partners...

https://www.citibank.com/dinersus/js...egory=fc79feb6

...and also seven hotel partners...

https://www.citibank.com/dinersus/js...egory=e350b05d

Note that Diners offers a recurring bonus on transfers to British Airways 20,000:30,000


Membership Rewards accumulate at American Express and there are 17 airline partners...

http://www.membershiprewards.com/cat...nd&sortDir=ASC

...and also four hotel partners...

http://www.membershiprewards.com/cat...gnized%3dfalse


Starpoints accumulate at Starwood and there are 30 airline partners...

http://www.starwoodhotels.com/prefer...tner_list.html

Note that the Starwood transfer ratio is only 20,000 : 12,500 to CO and UA.

Comparing Diners to the other two you should also factor in the wider acceptance of Mastercard verus American Express. The benefits provided by Diners Club are better than those offered with the SPG credit card, but if you already use Diners you will know if you actually utilize the concierge or insurance features.