From my experience: every promotion works. Most of the stat increases are quite modest, and won't have a huge impact on them.

Personally I prefer the attack upgrades. The infantry speed buff is the only stat increase that can have a big impact, both defensive and offensive. And the morale buff is especially good for healing with Heroic attacks, and for Pauline to spam her attack buff more often.

The heavy cavalry is more of a toss-up. I like the offensive so they're more likely to one-shot light cavalry, and have the accuracy to help against light infantry. But the defense buff makes them more reliable when slugging it out with heavy infantry or cavalry.