I've got the DL AmEx offer in my e-mail yesterday. $95 annual fee wiaved the first year, and a $99 companion domestic ticket. Limited blackout dates. (I believe)

The AS Visa is $75 fee and $99 companion ticket. So the difference is minimal. As above poster suggested, which airline has the routings you use most or are more favorable to you? That should be the decision maker. I might sign up for the DL card as it comes with 25k miles, and I fly DL occassionaly.