It really depends. I like having miles in both programs. If you are not going to attain status on either airline or if you can gain status on one airline, but still have some flying that year, but not enough to gain the next tier level of status, you can split your earning between the two with little downside.
You can only book DL flights using AS miles at the low level. Same with booking AS flights using DL miles - only AS flights at the low level. Also, DL miles are not good on "partners of partners." So if I want to fly ERA Aviation for example, I can do that with AS miles but not with DL miles. Each program also has different rules regarding stopovers and open jaws. I find it very beneficial to have miles in both programs.
good stuff here. thanks.
so say when i book a flight, i just take the confirmation or eticket number and add it to each of my programs? or can one booking only be "redeemed" with one program?