Which program to join. Huh. Well, you could do what I've done and what I suspect many other, if not most, FTers have done... join them both.

They're free; Join all the programs! If you join you'll be on the e-mail lists for special promos that might help you.

Perhaps the crux of your question really relates to recommendations on which program in which to accumulate miles? If so, know that of the two, DL and CO, only DL gives system wide upgrades with higher status, but if you read related posts you'll see that they can be tough to redeem. Still, you do get them. Careful as well: you can fly on DL and CO and accumlate OP miles but you might get dinged on the earning rate (50% for cheap fares); so DL is more generous there too.

If you wonder what program offers more "Complimentary Upgrades" based on status it really seems to vary based on which airport you fly from. Search and find some people love CO. If you search and find from your airport CO flyers get upgraded, great! Choose CO. Here's my experience, flying from SNA:

Click here for a post comparing CO and DL