Bear in mind that any site which advertises lists of "recommended" credit cards is likely to be earning an "affliate" referral fee which may influence the recomendations.

Starwood Preferred Guest credit card is useful for earning airline miles even if you never stay at a Starwood property. I wouldn't let your loyalty to Marriott discourage you from using it. Diners Club Mastercard is an excellent companion to the SPG card because it transfers to many of the same airlines and is accepted by more merchants, but overall SPG delivers more miles to more airlines for a lower annual fee.