I'd say with Battle Co. that the free units are a bigger deal than the ObSec (though ObSec Centurions are really annoying). With the War Convocation it does, as you say, make their little units perform better than you'd expect them to, but mostly just makes up for them being overpriced to begin with. I'd also say the 'tax units' are a lot bigger proportion of the list with Convocation than with Battle Co. It's getting the Canticles bonus with all the little Skitarii units that really breaks it out (and adding Pods, as Mike said).
But both are pretty strong in their own ways.
I would trade free hunter killer missiles for obsec everything any day.
"Don't take my win from me! I don't have many to claim!"
-Dandy