As a Platinum on DL and CO, I'd have to say both carriers are at parity now. There's some subtle +'s and -'s here and there, but overall, I think each carrier provides a comparable product in the air and on the ground.

So much so that when it comes to booking tickets now, I now choose the less expensive of the two. Two-three years ago, I would have selected CO exclusively because I felt at that time being a Platinum actually mattered and made a difference, and felt that the in-flight product was far better than DL's.