SPG is much better. you get more points -- 20K SPG = 25 DL, and they are more liquid. Accumulate at SPG and use them at SPG (easier than on an airline) or transfer to DL or most other airlines. Its a great way to top off dormant accounts. Amex fee is lower too. Only advantage of DL I see is the sign up bonus is richer the first year. On top of this, you don't have to worry about SPG filing BR.