DE is a much more aggressive game imo. A lot of things ultrakill does it does for the rule of cool, like the marksman coin ability, but doesnt do as much to foster aggression like DE does. This is probably a biased opinion though since ive only just completed act 2 in normal mode for the first time, while Ive done several nightmare clears of DE's campaign and dlc. The potential DPS you can reach in ultrakill is comparable to DE once you master the full kit, but reaching that mastery seems to be much faster in ultrakill.

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Basically DE is much more of a sweaty try hard's game while ultrakill is more content to do things just because its cool and not as laser focused on forcing the player to master every aspect of the combat system. Yeah its awesome you can parry your own shotgun blasts or throw coins to get a guaranteed hit on weakpoints, but its not particularly balanced, while DE is far more about having a balanced experience that has a very high skill ceiling and skill floor.

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Ofc you can do direct comparisons between things such as enemy variety and boss encounters, but its more interesting to me to compare the design philosophies behind the combat systems because that truly dictates how a game "feels". Given the budgets especially, i dont think its entirely fair to compare something like enemy variety when DE has the funding of the Bethesda and zenimax behind it while Ultrakill not only has a smaller budget, but also a much smaller dev team. To its credit, i do prefer ultrakill bosses to DE bosses in most cases.