So it's really a three part question:
Are you planning to field them solo?
Do you care about power in a competitive environment or a casual one?
Do you hate using Forgeworld options? (Even kitbashed stand-ins)
The second one is easier to answer, which is both of them do alright in competitive with allies, but only one or two mono-builds are propping up regular codex stuff at the top end, and without those mono-builds they end up about the same.
The first, if you're open to using allies, deathwatch has significantly more milage because of its flexibility, and you can run a "mono deathwatch" paint scheme as long as you differentiate which units are from a codex ally (I do this with
DA scouts and a land speeder vengeance)
The last is really the most important question, as pure codex deathwatch lack a lot of very important tools, that they can fill in with some of the dreadnought and armor options from the
FW index. Notably Seige Dreads, Chaplain Dreads, Mortis, C-mortis, and Leviathan.
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They're good in the sense that they're one of the only imperial units left with a form of advanced deployment after the recent Big
FAQ nerf hammering to strike from the shadows and similar things. With good placement they can cut off a lot of the melee tie-up shenanigans, including the stuff with fly now, which is a hugely important thing for most marine armies with how shooting heavy they are. Especially Gulliman's Drive-in as the tanks don't get the UM fallback tactics.
