Diners Club is a Mastercard. If the merchant accepts Mastercard they accept it, no exceptions.
Diners Club Rewards is more flexible than most Mastercard or VISA cards because you are not tied to a particular airline or hotel chain for reward redemptions. Diners Club is owned by Citibank, but it operates separately.
I believe Singlemalt was referring to the cards tied to the American Airlines' AAdvantage frequent flyer program, which are also issued by Citibank.
starwood amex? What am I gaining besides better rates for Starwood hotels?
I'm not sure what you mean by "better rates".
When you make a purchase with the Starwood credit card you earn one "Starpoint" for each dollar. Starpoints can be redeemed for award nights at Starwood hotel properties. Starpoints can also be transferred to airline frequent flyer programs. You redeem 20,000 Starpoints and (typically) receive 25,000 airline miles. This is better than Membership Rewards in which 20,000 points gives you only 20,000 miles. Then you redeem the airline miles for award tickets.
Inasmuch as you are in Pittsburgh it's worth nothing that Starpoints can be transferred to US Air, while Membership Rewards and Diners Club Rewards cannot.