I like DL much better. I don't do great sleeping on planes so I tend to take naps here and there. The fact there really isn't much a recline option on the VS planes is a dealbreaker. It's either seat or bed. Plus you physically have to get out of your seat to turn it into a bed. Very, very annoying.
Flight crew on DL is always usually pretty friendly but on VS can border on a bit snobby IMHO. Catering is different, but probably about the same quality on both.