Ok, I bought the game, but most of my time has been spent with the Fallout mod. From what I've gathered, the strict unit limits from the commanders means that quality is preferable to quantity. A single offensive can reasonably only have 36 units, so attachments make sense in the long run.
As for attachments, our artillery does not slow down our regular infantry, but halves the speed of our motorized units. Ergo, they're great for regular infantry but lowsy for motorized.
And is it just me, or are the starting armored cars better than the starting light tanks as attachments, in general? Granted, the armored car doesn't reduce the softness score as much as the light tank, but in all other ways the stat boost is superior for the armored car.
It also appears that the Char B medium tank is equal or even superior in every capability to the Char D heavy tank as a motorized infantry attachment, and the medium tank doesn't slow our motorized infantry unlike the heavy. So I'd go entirely with medium tank brigades attached to the motorized units.
Plus, medium tanks have the advantage of more opportunities to upgrade, unlike the armored cars.
I also booted up the 1940 scenario, in which France had a couple tank divisions. Their stats are jaw dropping awesome, and I'm glad I pushed for them in this playthrough.
For our next technology, we need escort fighters! While our bombers are lousy, they won't survive to deliver their pitiful payloads without escorts.