Probably depends on where you're traveling. Personally, I picked CO, but EWR is the most convenient airport for me, so AA, DL, etc., were really not good options.

The credit card choice is secondary. Might as well get one that matches the program you commit to.

As to which is easier to maintain, depends on where you go and on whose metal. Dl now has rollover miles which might make it easier, unless of course you find 30% of your travel on *A vs. ST.