I'm not much for airline credit cards and feel that hotel cards offer better value. However, there is certainly some value to 30k DL miles. I would also note that the gap between DL and AA on availability has closed substantially, primarily owing to a worsening availability situation among OW airlines. I can't speak much to UA, as I have little interest in flying UA.

If I were looking for an airline credit card, I would consider the BA card. The mileage bonuses are pretty decent and Avios can deliver pretty solid value for short-haul (both US and Europe), as well as upgrades on BA metal.