I say more activations, for various reasons based on yours.
Having more activations is an advantage in every game based on the "i move a unit, you move a unit" system. It allows you to see what your opponent does, to wait before playong your master unit etc etc.
It also allows you to attack more easily on various sides, and to keep pressure on the ennemy forces.
I would also say that you can have more minis on Sentry mode with little units than big, since even very small units can have 2 Sentires.
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3 difficult to break.
The Ld level of every models in the game is pretty high. As long as the squad commander of a squad lives, even small units are pretty hard to break, except for a few such as Heretics, which i doubt are much played nowadays.
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1 with single card or power you buff more model.
This is to me the only good point in big squads. But irrelevant compared to all the good ones for small squads.
All of this depends on your army and deck of course, but all in all i think it's generally better to spread the points of your army in many little forces.