With DL you might have better UG chances, especially since CO Plat will be third tier next year. You probably won't have E+ but there is no assurance yet from UA that it will continue anyway. Besides, it is still coach seating.

Also with DL, more opportunities for wi-fi and pretty good IROPS service for elites.

The biggest deserved knock for DL is award travel, especially intl. OTOH, it is easier to earn SkyMiles than OnePass. Even so, award travel can be pricey. For some, that is enough of a dealbreaker.

Product-wise, they are pretty similar but some say the food is better with CO and DL is known to have more flt delays. In the end, you choose the one that works best for you.