So back to the main topic I can see a lot of reasons that you'd pick various factions while Deathwatch don't technically exist.
Chaos Tzeentch gives you psybolts but you lose They Shall Know No fear.
Grey Knights give you Psibolters, and Stormbolters, as well as special CC gear, but the extra psychics are fine with your leader, but a bit out of place for the regular dude unless you consider it a Deathwatch 'tech' in effect. (IE: Astral Aim = Frag tech or something.)
Astartes Scouts. This one is the hardest for me personally, but you will get sniper rifles (Stalker Bolters), Heavy Bolter, Missile launcher, and Shotguns. The weird thing is 'Noviciates' and the carapace armour.
No word yet besides the one special op that scouts can take. Though GW has promised more teams on the way and we know rules for Sisters and Inq are incoming so maybe there's hope.
We've got 3 schools of thought going on in another thread.
1) Scouts as Kill Marines, pretty easy translation.
2) GK counts as can bring DW style firepower (storm bolters, psycannons, etc)
3) Chaos counts as works the best for the team I want to build. A MoK Champ makes a good Blackshield, and you get access to some of the special gear (camo cloaks, etc) that GKs can't get, so I can easily run a geared out sniper.
I missed the MoK which is a great Blackshield!