As someone who's put 100+ hours into both with friends, Darktide is better (now), for the simple reason that you have more abilities to play with in Darktide. I played as the pyromancer in Vermintide, and my options were generally limited to melee, ranged (ability varying based off chosen weapon), and ultimate. In Darktide, I play as the psyker and on top of melee and ranged I also have a pair of utility spells (of my choosing) that I can also mix up based off the situation. My particular psyker has ranged charged/non-charged bolts for primary DPS, chain lightning for crowd control, and a shield bubble that I can deploy at range to mitigate enemy fire. It's not as panic-inducing as being rushed down by rats in Vermintide, but I feel much more capable and satisfied for/by making tactical decisions.
The game's also just easier to play. The Vermintide gear system is an RNG nightmare but getting better gear and upgrading it in Darktide is a much more pleasant experience. Stage collectibles are also better in Darktide (by which I mean there aren't as many of them and they don't rely on janky platforming like in Vermintide).