I'm going to be running a Deathwatch game that incorporates a large amount of rules from Only War. I've decided that each Specialty will start with six Aptitudes. I'm also including Sergeant and Chaplain as available Specialties.

Please let me know if the Aptitudes seem appropriate in your opinion.

Tactical Marine - Ballistic Skill, Weapon Skill, Agility, Fellowship, Finesse, Offense

Assault Marine - Weapon Skill, Strength, Agility, Perception, Fieldcraft, Finesse

Devastator Marine - Ballistic Skill, Strength, Toughness, Perception, Defense, Offense

Sergeant - Ballistic Skill, Toughness, Agility, Fellowship, Defense, Leadership

Apothecary - Weapon Skill, Strength, Toughness, Intelligence, Fieldcraft, Knowledge

Techmarine - Ballistic Skill, Toughness, Intelligence, Willpower, Knowledge, Tech

Librarian - Weapon Skill, Intelligence, Perception, Willpower, Knowledge, Psyker

Chaplain - Weapon Skill, Perception, Willpower, Fellowship, Leadership, Social

I'm not quite asking if Deathwatch should be run with Aptitudes, but just to see if they look distributed appropriately. With the exception of Tech, Psyker, and Social, all the Characteristic Aptitudes appear 3-4 times and all the other Special Aptitudes appear twice.

Looks pretty good to me. I'm doing something similar, but more involved.

What about aptitude for chapters as an alternative?

Aptitudes for Chapters alone can't account for the differences between the different classes. The only way it could work is with a truly classless system, which would be a hard sell.