As someone who willingly left CO, I do not think that CO is a better option than DL.

- Upgrades are notoriously hard to snag.
- Redemptions are notoriously difficult; I almost always have to take a partner for at least some legs. Plat does not seem to help much, if at all.
- The 50% EQM thing means that you are stuck flying CO (=31" pitch and hard seats) or paying a premium. A large number of business travelers are flat-out shafted, as even CO tickets booked through TAs get the 50% treatment.
- Lack of flexibility during irregular ops is widely acknowledged, even by some ardent supporters.
- An EWR hub that is worse than ATL, with employees widely seen as surly and unhelpful.
- No PMUs of any kind.
- No MM program, though one is supposedly coming.
- No plans to upgrade their J product until, at least 2010, when the first 787 comes.
- TATL 752s are no fun.
- Phone agents are often woefully uninformed. Booking reward tickets can be a nightmare, unless you really know your stuff.

So, unless you really love airline food, or really like the idea of VS or EK redemptions, then I'm not sure that CO is all that.

If you travel in paid premium classes, though, I would agree that CO is great for domestic F, but as long as they are in ST, I'm not sure it makes much difference which program you accrue in.

Of course, many of the points above apply to DL, but many don't, so I think DL wins on the whole.