How much do you spend on cards in a typical year?
All of the Delta credit cards earn one Skymile per dollar on general spending. If you use an American Express card that earns Membership Rewards points you can earn more than one point per dollar in some categories. The points can be converted to Skymiles 1000=1000, with a small fee for the transfer, and the points can also be transferred to several other airline programs for those times when Delta is not the best choice to a particular destination. However, only the Delta cards will earn MQM's and waive the airfare spend requirement for Medallion status. (In the USA, all credit cards that earn Skymiles are issued by American Express.)
I generally spend somewhere around $1.5M on cards depending on where I'm at with business expansion. I use AmEx where I can solely for the airline mile benefit and have had few vendors deny it, in which case I use a FlexPerks Visa, which I redeem mostly for hotel and airfare. I've had that card for probably 25 years as it transitioned though various programs so I've just always kept it...another card I'm not sure is all that valuable. Those are the only two credit cards I have, period, so everything I do is on those. I do make some purchases through Delta when there are 4x-10x mile offers (computers, etc), but again it doesn't add up very quickly. I fly only 5-10 times a year and only use miles to do so, hence the travel reward cards if that makes a difference to what cards I might look at. Thank you for the help and information!